Search

We present X-ray and multiband optical observations of the afterglow and host galaxy of GRB 180418A, discovered by Swift/BAT and Fermi/GBM. We present a reanalysis of the GBM and BAT data deriving durations of the prompt emission of T 90 ≈ 2.56 and 1.90 s, respectively. Modeling the Fermi/GBM catalog of 1405 bursts (2008–2014) in the hardness–T 90 plane, we obtain a probability of ≈60% that GRB 180418A is a short-hard burst. From a combination of Swift/XRT and Chandra observations, the X-ray afterglow is detected to ≈38.5 days after the burst and exhibits a single power-law decline with F X ∝ t −0.98. Late-time Gemini observations reveal a faint r ≈ 25.69 mag host galaxy at an angular offset of ≈0.″16. At the likely redshift range of z ≈ 1–2.25, we find that the X-ray afterglow luminosity of GRB 180418A is intermediate between short and long gamma-ray bursts (GRBs) at all epochs during which there are contemporaneous data and that GRB 180418A lies closer to the E γ,peak–E γ,iso correlation for short GRBs. Modeling the multiwavelength afterglow with the standard synchrotron model, we derive the burst explosion properties and find a jet opening angle of θ j ≳ 9°–14°. If GRB 180418A is a short GRB that originated from a neutron star merger, it has one of the brightest and longest-lived afterglows along with an extremely faint host galaxy. If, instead, the event is a long GRB that originated from a massive star collapse, it has among the lowest-luminosity afterglows and lies in a peculiar space in terms of the hardness–T 90 and E γ,peak–E γ,iso planes.

Full list of authors: Rossi, A.; Frederiks, D. D.; Kann, D. A.; De Pasquale, M.; Pian, E.; Lamb, G.; D'Avanzo, P.; Izzo, L.; Levan, A. J.; Malesani, D. B.; Melandri, A.; Guelbenzu, A. Nicuesa; Schulze, S.; Strausbaugh, R.; Tanvir, N. R.; Amati, L.; Campana, S.; Cucchiara, A.; Ghirlanda, G.; Della Valle, M.; Klose, S.; Salvaterra, R.; Starling, R. L. C.; Stratta, G.; Tsvetkova, A. E.; Vergani, S. D.; D'Ai, A.; Burgarella, D.; Covino, S.; D'Elia, V; Postigo, A. de Ugarte; Fausey, H.; Fynbo, J. P. U.; Frontera, F.; Guidorzi, C.; Heintz, K. E.; Masetti, N.; Maiorano, F.; Mundell, C. G.; Oates, S. R.; Page, M. J.; Palazzi, E.; Palmerio, J.; Pugliese, G.; Rau, A.; Saccardi, A.; Sbarufatti, B.; Svinkin, D. S.; Tagliaferri, G.; van der Horst, A. J.; Watson, D. J.; Ulanov, M., V; Wiersema, K.; Xu, D.; Zhang, J.--This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. Following the detection by Swift and Konus-Wind, we obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27−0.19+0.20 × 1054 erg, GRB 210905A lies in the top ∼7% of gamma-ray bursts (GRBs) in the Konus-Wind catalogue in terms of energy released. Its afterglow is among the most luminous ever observed, and, in particular, it is one of the most luminous in the optical at t ≳ 0.5 d in the rest frame. The afterglow starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ∼46.2 ± 16.3 d (6.3 ± 2.2 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to a constant contribution from the host galaxy and potentially from a foreground intervening galaxy. In particular, the host galaxy is only the fourth GRB host at z > 6 known to date. By assuming a number density n = 1 cm−3 and an efficiency η = 0.2, we derived a half-opening angle of 8.4 ° ±1.0°, which is the highest ever measured for a z ≳ 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of ≃1 × 1052 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent within 2σ with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift. © A. Rossi et al. 2022., A. Rossi acknowledges support from the INAF project Premiale Supporto Arizona & Italia. D.D.F. and A.E.T. acknowledge support from RSF grant 21-12-00250. D.A.K. acknowledges support from Spanish National Research Project RTI2018-098104-J-I00 (GRBPhot). A.R., E.Pal., P.D.A., L.A., E.Pi., G.S., S.C., V.D.E., M.D.V., and A.M. acknowledge support from PRIN-MIUR 2017 (grant 20179ZF5KS). P.D.A., A.M. acknowledge support from the Italian Space Agency, contract ASI/INAF n. I/004/11/5. L.I. was supported by grants from VILLUM FONDEN (project number 16599 and 25501). D.B.M. and A.J.L. acknowledge the European Research Council (ERC) under the European Union’s Seventh Framework programme (FP7-2007-2013) (grant agreement No. 725246). The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant No. 140. K.E.H. acknowledges support by a Postdoctoral Fellowship Grant (217690–051) from The Icelandic Research Fund. C.G.M. acknowledges financial support from Hiroko and Jim Sherwin. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

The typical detection rate of $\sim1$ gamma-ray burst (GRB) per day by the \emph{Fermi} Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the \emph{Fermi} localization typically prevents rapid identification of multi-wavelength counterparts. We report the follow-up of 93 \emph{Fermi} GRBs with the Gravitational-wave Optical Transient Observer (GOTO) prototype on La Palma. We selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. We apply a filtering process consisting of both automated and manual steps to 60\,085 candidates initially, rejecting all but 29, arising from 15 events. With $\approx3$ GRB afterglows expected to be detectable with GOTO from our sample, most of the candidates are unlikely to be related to the GRBs. Since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the GRBs. Our results show that GOTO can effectively search for GRB optical counterparts thanks to its large field of view of $\approx40$ square degrees and its depth of $\approx20$ mag. We also detail several methods to improve our overall performance for future follow-up programs of \emph{Fermi} GRBs.

The number of long gamma-ray bursts (GRBs) known to have occurred in the distant Universe (z greater than 5) is small (approx 15), however these events provide a powerful way of probing star formation at the onset of galaxy evolution. In this paper, we present the case for GRB100205A being a largely overlooked high-redshift event. While initially noted as a high-z candidate, this event and its host galaxy have not been explored in detail. By combining optical and near-infrared Gemini afterglow imaging (at t less than 1.3 days since burst) with deep late-time limits on host emission from the Hubble Space Telescope, we show that the most likely scenario is that GRB100205A arose in the redshift range 4-8. GRB100205A is an example of a burst whose afterglow, even at 1 hour post-burst, could only be identified by 8m class IR observations, and suggests that such observations of all optically dark bursts may be necessary to significantly enhance the number of high-redshift GRBs known., Accepted for publication in MNRAS

We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, θc. We fit model and observed afterglow light curves with a smoothly-broken power-law function with a free-parameter κ that describes the sharpness of the break. We find that the jet break is sharper (κ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of 0.1 ≤ κ ≤ 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 ≤ κ ≤ 2.2 when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within θc, whereas for smooth-edged jets, κ is more strongly dependent on the size of θc. Using a sample of 20 GRBs, we find 9 candidate smooth-edged jet structures and 8 candidate sharp-edged jet structures, while the remaining 3 are consistent with either. The shape of the jet break, as measured by the sharpness parameter κ, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time. © 2021 The Author(s)., GPL and JJF thank Shiho Kobayashi for useful discussions and comments on a pre-submission version. GPL thanks Lekshmi Resmi for valuable discussions. GPL is supported by the Science Technology and Facilities Council (STFC) via grant ST/S000453/1. DAK acknowledges support from Spanish National Research Project RTI2018-098104-J-I00 (GRBPhot), and thanks S. Schulze and M. Blažek for calculations support. IM is a recipient of the Australian Research Council Future Fellowship FT190100574., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

The discovery of GW170817 and GRB 170817A in tandem with AT 2017gfo cemented the connection between neutron star mergers, short gamma-ray bursts (GRBs), and kilonovae. To investigate short GRB observations in the context of diverse kilonova behavior, we present a comprehensive optical and near-infrared (NIR) catalog of 85 bursts discovered over 2005-2020 on timescales of $\lesssim12$ days. The sample includes previously unpublished observations of 23 bursts, and encompasses both detections and deep upper limits. We identify 11.8% and 15.3% of short GRBs in our catalog with upper limits that probe luminosities lower than those of AT 2017gfo and a fiducial NSBH kilonovae model (for pole-on orientations), respectively. We quantify the ejecta masses allowed by the deepest limits in our catalog, constraining blue and `extremely blue' kilonova components of 14.1% of bursts to $M_{\rm ej}\lesssim0.01-0.1 M_{\odot}$. The sample of short GRBs is not particularly constraining for red kilonova components. Motivated by the large catalog as well as model predictions of diverse kilonova behavior, we investigate modified search strategies for future follow-up to short GRBs. We find that ground-based optical and NIR observations on timescales of $\gtrsim 2$ days can play a significant role in constraining more diverse outcomes. We expect future short GRB follow up efforts, such as from the {\it James Webb Space Telescope}, to expand the reach of kilonova detectability to redshifts of $z\approx 1$., Comment: 32 pages, 7 figures, 2 tables. Submitted to ApJ

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Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.--Full list of authors: Ackley, K.; Amati, L.; Barbieri, C.; Bauer, F. E.; Benetti, S.; Bernardini, M. G.; Bhirombhakdi, K.; Botticella, M. T.; Branchesi, M.; Brocato, E.; Bruun, S. H.; Bulla, M.; Campana, S.; Cappellaro, E.; Castro-Tirado, A. J.; Chambers, K. C.; Chaty, S.; Chen, T. -W.; Ciolfi, R.; Coleiro, A.; Copperwheat, C. M.; Covino, S.; Cutter, R.; D'Ammando, F.; D'Avanzo, P.; De Cesare, G.; D'Elia, V.; Della Valle, M.; Denneau, L.; De Pasquale, M.; Dhillon, V. S.; Dyer, M. J.; Elias-Rosa, N.; Evans, P. A.; Eyles-Ferris, R. A. J.; Fiore, A.; Fraser, M.; Fruchter, A. S.; Fynbo, J. P. U.; Galbany, L.; Gall, C.; Galloway, D. K.; Getman, F. I.; Ghirlanda, G.; Gillanders, J. H.; Gomboc, A.; Gompertz, B. P.; González-Fernández, C.; González-Gaitán, S.; Grado, A.; Greco, G.; Gromadzki, M.; Groot, P. J.; Gutiérrez, C. P.; Heikkilä, T.; Heintz, K. E.; Hjorth, J.; Hu, Y. -D.; Huber, M. E.; Inserra, C.; Izzo, L.; Japelj, J.; Jerkstrand, A.; Jin, Z. P.; Jonker, P. G.; Kankare, E.; Kann, D. A.; Kennedy, M.; Kim, S.; Klose, S.; Kool, E. C.; Kotak, R.; Kuncarayakti, H.; Lamb, G. P.; Leloudas, G.; Levan, A. J.; Longo, F.; Lowe, T. B.; Lyman, J. D.; Magnier, E.; Maguire, K.; Maiorano, E.; Mandel, I.; Mapelli, M.; Mattila, S.; McBrien, O. R.; Melandri, A.; Michałowski, M. J.; Milvang-Jensen, B.; Moran, S.; Nicastro, L.; Nicholl, M.; Nicuesa Guelbenzu, A.; Nuttal, L.; Oates, S. R.; O'Brien, P. T.; Onori, F.; Palazzi, E.; Patricelli, B.; Perego, A.; Torres, M. A. P.; Perley, D. A.; Pian, E.; Pignata, G.; Piranomonte, S.; Poshyachinda, S.; Possenti, A.; Pumo, M. L.; Quirola-Vásquez, J.; Ragosta, F.; Ramsay, G.; Rau, A.; Rest, A.; Reynolds, T. M.; Rosetti, S. S.; Rossi, A.; Rosswog, S.; Sabha, N. B.; Sagués Carracedo, A.; Salafia, O. S.; Salmon, L.; Salvaterra, R.; Savaglio, S.; Sbordone, L.; Schady, P.; Schipani, P.; Schultz, A. S. B.; Schweyer, T.; Smartt, S. J.; Smith, K. W.; Smith, M.; Sollerman, J.; Srivastav, S.; Stanway, E. R.; Starling, R. L. C.; Steeghs, D.; Stratta, G.; Stubbs, C. W.; Tanvir, N. R.; Testa, V.; Thrane, E.; Tonry, J. L.; Turatto, M.; Ulaczyk, K.; van der Horst, A. J.; Vergani, S. D.; Walton, N. A.; Watson, D.; Wiersema, K.; Wiik, K.; Wyrzykowski, Ł.; Yang, S.; Yi, S. -X.; Young, D. R., Context. Gravitational wave (GW) astronomy has rapidly reached maturity, becoming a fundamental observing window for modern astrophysics. The coalescences of a few tens of black hole (BH) binaries have been detected, while the number of events possibly including a neutron star (NS) is still limited to a few. On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. A preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. Aims. In this paper, we present our extensive search campaign aimed at uncovering the potential optical and near infrared electromagnetic counterpart of S190814bv. We found no convincing electromagnetic counterpart in our data. We therefore use our non-detection to place limits on the properties of the putative outflows that could have been produced by the binary during and after the merger. Methods. Thanks to the three-detector observation of S190814bv, and given the characteristics of the signal, the LIGO and Virgo Collaborations delivered a relatively narrow localisation in low latency - a 50% (90%) credible area of 5 deg2 (23 deg2) - despite the relatively large distance of 267 ± 52 Mpc. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical and near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS, and VINROUGE projects also carried out a search on this event. In this paper, we describe the combined observational campaign of these groups. Results. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN), which was possibly generated by this NS-BH merger, and for the strategy of future searches. The typical depth of our wide-field observations, which cover most of the projected sky localisation probability (up to 99.8%, depending on the night and filter considered), is r ∼ 22 (resp. K ∼ 21) in the optical (resp. near infrared). We reach deeper limits in a subset of our galaxy-targeted observations, which cover a total ∼50% of the galaxy-mass-weighted localisation probability. Altogether, our observations allow us to exclude a KN with large ejecta mass M 0.1 M- to a high (> 90%) confidence, and we can exclude much smaller masses in a sub-sample of our observations. This disfavours the tidal disruption of the neutron star during the merger. Conclusions. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv, we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundred megaparsecs will be detected only by large facilities with both a high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event. © K. Ackley et al. 2020., Based on observations collected at the European Southern Observatory under ESO programmes 1102.D-0353(E), 1102.D0353(F), 1102.D-0353(Q), 1102.D-0353(G), 0103.D-0070(A), 0103.D-0070(B), 0103.D-0703(A), 0103.D-0722(A), 0103.A-9099(A), 198.D-2010(D) and 60.A9285(A). ATLAS is primarily funded through NEO NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575. The ATLAS science products have been made possible through the contributions of the University of Hawaii IfA, the Queen's University Belfast, the Space Telescope Science Institute, and the South African Astronomical Observatory. PanSTARRS is primarily funded through NEO NASA grants NASA Grants NNX08AR22G, NNX14AM74G. The PanSTARRS science products for LIGO-Virgo follow-up are made possible through the contributions of the University of Hawaii IfA and the Queen's University Belfast. The Gravitational-wave Optical Transient Observer (GOTO) project acknowledges the support of the Monash-Warwick Alliance; Warwick University; Monash University; She ffield University; Leicester University; Armagh Observatory & Planetarium; the National Astronomical Research Institute of Thailand (NARIT); University of Portsmouth; Turku University and the Instituto de Astrofisica de Canarias (IAC). Part of the funding for GROND was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. The WHT and its override programme are operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias; part of these data were taken under program (19A)N3. FEB thanks CONICYT Basal AFB-170002 and Chile's Ministry of Economy fund IC120009. MGB, PDA and AM acknowledge support from ASI grant I/004/11/3. MBr, EC, AP and SPi acknowledge support from MIUR (PRIN 2017 grant 20179ZF5KS). EB, EM and MT acknowledge funding from GRAWITA. SHB is indebted to the Danish National Research Foundation (DNRF132) for support. SCa acknowledges support from grant MAE0065741. EC acknowledges the support of the H2020 OPTICON programme 730890. TWC acknowledges the Humboldt Foundation and Marie Sklodowska-Curie grant 842471. MDP thanks Istanbul University for support. PAE acknowledges UKSA support. RAJEF is supported by an STFC studentship. MF is supported by a Royal Society -SFI University Research Fellowship. LG was funded by the EU H2020 programme under MSCA grant no. 839090. CG, JH and LI were supported by a research grant from VILLUM FONDEN (project 16599). CG and LI were supported by a research grant from VILLUM FONDEN (25501). GGh acknowledges the PRIN MIUR "Figaro" for financial support. AGo acknowledges financial support from the Slovenian Research Agency (grants P1-0031, I0-0033, and J1-8136). BPG, AJL and JDL acknowledge support from ERC grant 725246 (TEDE, PI Levan). SGG acknowledges support by FCT Fundacao para a Ciencia e Tecnologia and by Project PTDC/FIS-AST-31546. GGr acknowledges the ESCAPE H2020 project no. 824064. MG is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. PJG acknowledges support from NOVA and from the South African NRF SARChI grant 111692. CPG and MS acknowledge support from EU/FP7-ERC grant no. 615929. KEH acknowledges support by a Project Grant from The Icelandic Research Fund. YDH acknowledges support from the China Scholarships Council. JJ acknowledges support from NOVA and NWO-FAPESP grant for instrumentation. AJ acknowledges funding from the European Research Council (ERC). ZPJ was supported by the Foundation for Distinguished Young Scholars of Jiangsu Province (no. BK20180050). PGJ acknowledges funding from the ERC under Consolidator Grant agreement no. 647208. DAK acknowledges Spanish research project RTI2018-098104-J-I00 (GRBPhot). SKl acknowledges support by DFG grant Kl 766/16-3. ECK acknowledges support from the GREAT research environment. GPL acknowledges support from STFC via grant ST/N000757/1. GL was supported by a research grant (19054) from VILLUM FONDEN. KM acknowledges support from the ERC (grant no. 758638). IM is partially supported by OzGrav (ARC project CE17010000). MMacknowledges support from ERC through ERC-2017-CoG no. 770017. MJM acknowledges the National Science Centre, Poland, grant 2018/30/E/ST9/00208. BMJ and DW are supported by Independent Research Fund Denmark grant DFF-7014-00017. MN is supported by a Royal Astronomical Society Research Fellowship. ANG acknowledges support by grant DFG Kl 766/16-3. PTOB acknowledges funding from STFC. SRO gratefully acknowledges the support of the Leverhulme Trust. FO acknowledges the support of the H2020 Hemera program, grant no. 730970. MAPT was supported by grants RYC-2015-17854 and AYA201783216-P. EP aknowledges financial support from INAF. GP is supported by the Millennium Science Initiative through grant IC120009. MLP is partially supported by a "Linea 2" project of the Catania University. JQV acknowledges support from CONICYT folio 21180886. TMR acknowledges the support of the Vilho, Yrjo and Kalle Vaisala Foundation. ARo acknowledges support from Premiale LBT 2013. SR is supported by VR grants 2016-03657_3 and the research environment grant GREAT, Dnr. 2016-06012, and the Swedish National Space board, Dnr. 107/16. OSS acknowledges the Italian Ministry of Research (MIUR) grant 1.05.06.13. LSa acknowledges the Irish Research Council Scholarship no. GOIPG/2017/1525. SJS acknowledges support from STFC Grant ST/P000312/1. ERS and DS acknowledge funding from UK STFC CG ST/P000495/1. RLCS acknowledges funding from STFC. DS acknowledges support from STFC via grant ST/T007184/1. SDV acknowledges the support of the CNES. LWsupported by Polish NCN DAINA 2017/27/L/ST9/03221. The Cosmic DAWN center is funded by the Danish National Research Foundation.

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. The identification of the sources that reionized the Universe and their specific contribution to this process are key missing pieces of our knowledge of the early Universe. Faint star-forming galaxies may be the main contributors to the ionizing photon budget during the epoch of reionization, but their escaping photons cannot be detected directly due to inter-galactic medium opacity. Hence, it is essential to characterize the properties of faint galaxies with significant Lyman continuum (LyC) photon leakage up to z ∼ 4 to define indirect indicators allowing analogs to be found at the highest redshift. Aims. Long gamma-ray bursts (LGRBs) typically explode in star-forming regions of faint, star-forming galaxies. Through LGRB afterglow spectroscopy it is possible to detect directly LyC photons. Our aim is to use LGRBs as tools to study LyC leakage from faint, star-forming galaxies at high redshift. Methods. Here we present the observations of LyC emission in the afterglow spectra of GRB 191004B at z = 3.5055, together with those of the other two previously known LyC-leaking LGRB host galaxies (GRB 050908 at z = 3.3467, and GRB 060607A at z = 3.0749), to determine their LyC escape fraction and compare their properties. Results. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of log(NH I/cm−2) = 17.2 ± 0.15, and negligible extinction (AV = 0.03 ± 0.02 mag). The only metal absorption lines detected are C IV and Si IV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Lyman-alpha (Lyα) emission. From its Lyα emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to 1 ≤ SFR ≤ 4.7 M⊙ yr−1. We fit the Lyα emission with a shell model and find parameters values consistent with the observed ones. The absolute (relative) LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of 0.35−0.11+0.10 (0.43−0.13+0.12), 0.08−0.04+0.05 (0.08−0.04+0.05) and 0.20−0.05+0.05 (0.45−0.15+0.15), respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRBs presented here have all M1600 > −19.5 mag, with the GRB 060607A host at M1600 > −16 mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore, the time involved is minimal compared to galaxy studies. © J.-B. Vielfaure et al. 2020, This work is part of the BEaPro project (PI: S.D. Vergani) funded by the French Agence Nationale de la Recherche (ANR-16-CE31-0003). We thank Giancarlo Ghirlanda for providing useful information. JBV and SDV thank Anne Verhamme for useful discussions. SDV acknowledges financial support from the French Space Agency (CNES). MG was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51409 and acknowledges support from HST grants HST-GO-15643.017-A, HST-AR-15039.003-A, and XSEDE grant TG-AST180036. The Cosmic DAWN center is funded by the DNRF. JPUF thanks the Carlsberg foundation for support. DBM acknowledges support from VILLUM FONDEN research grant 19054. NRT acknowledges support from STFC via grant ST/N000757/1. DAK acknowledges support from Spanish research project RTI2018-098104-J-I00 (GRBPhot). The PanSTARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project O ffice, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The NumPy (van der Walt et al. 2011), SciPy (Virtanen et al. 2020) and matplotlib (Hunter 2007) packages have been extensively used for the preparation and presentation of this work.

Short gamma-ray bursts (SGRBs) are now known to be the product of the merger of two compact objects. However, two possible formation channels exist: neutron star -- neutron star (NS -- NS) or NS -- black hole (BH). The landmark SGRB 170817A provided evidence for the NS -- NS channel, thanks to analysis of its gravitational wave signal. We investigate the complete population of SGRBs with an associated redshift (39 events), and search for any divisions that may indicate that a NS -- BH formation channel also contributes. Though no conclusive dichotomy is found, we find several lines of evidence that tentatively support the hypothesis that SGRBs with extended emission (EE; 7 events) constitute the missing merger population: they are unique in the large energy band-sensitivity of their durations, and have statistically distinct energies and host galaxy offsets when compared to regular (non-EE) SGRBs. If this is borne out via future gravitational wave detections it will conclusively disprove the magnetar model for SGRBs. Furthermore, we identify the first statistically significant anti-correlation between the offsets of SGRBs from their host galaxies and their prompt emission energies., 23 pages, 9 figures, 2 tables. 1 appendix (2 pages, 1 table). Author's final accepted version, to be published in ApJ

Context. For the first time, very high energy emission up to the TeV range has been reported for a gamma-ray burst (GRB). It is still unclear whether the environmental properties of GRB 190114C might have contributed to the production of these very high energy photons, or if it is solely related to the released GRB emission. Aims. The relatively low redshift of the GRB (z = 0.425) allows us to study the host galaxy of this event in detail, and to potentially identify idiosyncrasies that could point to progenitor characteristics or environmental properties that might be responsible for this unique event. Methods. We used ultraviolet, optical, infrared, and submillimetre imaging and spectroscopy obtained with the HST, the VLT, and ALMA to obtain an extensive dataset on which the analysis of the host galaxy is based. Results. The host system is composed of a close pair of interacting galaxies (Δv = 50 km s-1), both of which are well detected by ALMA in CO(3-2). The GRB occurred within the nuclear region (∼170 pc from the centre) of the less massive but more star-forming galaxy of the pair. The host is more massive (log(M/M⊙ ) = 9.3) than average GRB hosts at this redshift, and the location of the GRB is rather unique. The higher star formation rate was probably triggered by tidal interactions between the two galaxies. Our ALMA observations indicate that both host galaxy and companion have a high molecular gas fraction, as has been observed before in interacting galaxy pairs. Conclusions. The location of the GRB within the core of an interacting galaxy with an extinguished line of sight is indicative of a denser environment than typically observed for GRBs and could have been crucial for the generation of the very high energy photons that were observed. © ESO 2020., This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.A.00020.T. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. CT, AdUP, DAK, MB, and LI acknowledge support from the Spanish research project AYA2017-89384-P. CT and AdUP acknowledge support from funding associated to Ramon y Cajal fellowships (RyC-2012-09984 and RyC-2012-09975). DAK acknowledges support from the Spanish research project RTI2018-098104-J-I00. M.J.M. acknowledges the support of the National Science Centre, Poland through the SONATA BIS grant 2018/30/E/ST9/00208. JPUF thanks the Carlsberg Foundation for support. KEH and PJ acknowledge support by a Project Grant (162948-051) from The Icelandic Research Fund. JS and DW are supported in part by Independent Research Fund Denmark grant DFF-7014-00017. The Cosmic Dawn Center is supported by the Danish National Research Foundation under grant No. 140.

We present the first spatially resolved study of molecular gas in the vicinity of a Gamma Ray Burst, using CO(2-1) emission line observations with the Atacama Large Millimetre Array (ALMA) at ~50 pc scales. The host galaxy of GRB 980425 contains a ring of high column density HI gas which is likely to have formed due to a collision between the GRB host and its companion galaxy, within which the GRB is located. We detect eleven molecular gas clumps in the galaxy, seven of which are within the gas ring. The clump closest to the GRB position is at a projected separation of ~280 pc. Although it is plausible that the GRB progenitor was ejected from clusters formed in this clump, we argue that the in situ formation of the GRB progenitor is the most likely scenario. We measure the molecular gas masses of the clumps and find them to be sufficient for forming massive star clusters. The molecular gas depletion times of the clumps show a variation of ~2 dex, comparable with the large variation in depletion times found in starburst galaxies in the nearby Universe. This demonstrates the presence of starburst modes of star formation on local scales in the galaxy, even while the galaxy as a whole cannot be categorised as a starburst based on its global properties. Our findings suggest that the progenitor of GRB 9802425 was originated in a young massive star cluster formed in the starburst mode of star formation., Comment: ApJ (in press), 12 pages, 4 figures, 1 table

We present post-jet-break Hubble Space Telescope, Very Large Array, and Chandra observations of the afterglow of the long gamma-ray bursts GRB 160625B (between 69 and 209 days) and GRB 160509A (between 35 and 80 days). We calculate the post-jet-break decline rates of the light curves and find the afterglow of GRB 160625B is inconsistent with a simple t(-3/4) steepening over the break, expected from the geometric effect of the jet edge entering our line of sight. However, the favored optical post-break decline is also inconsistent with the f(nu) proportional to t(-p) decline (where p 2.3 from the pre-break light curve), which is expected from exponential lateral expansion of the jet; perhaps suggesting lateral expansion that only affects a fraction of the jet. The post-break decline of GRB 160509A is consistent with both the t(-3/4) steepening and with f(nu) proportional to t(-p). We also use boxfit to fit afterglow models to both light curves and find both to be energetically consistent with a millisecond magnetar central engine, but the magnetar parameters need to be extreme (i.e., E similar to 3 x 10(52) erg). Finally, the late-time radio light curves of both afterglows are not reproduced well by boxfit and are inconsistent with predictions from the standard jet model; instead, both are well represented by a single power-law decline (roughly f(nu) proportional to t(-1)) with no breaks. This requires a highly chromatic jet break and possibly a two-component jet for both bursts.© 2020. The American Astronomical Society. All rights reserved., A.C. acknowledges support from the National Science Foundation via CAREER award #1455090. A.d.U.P. acknowledges support from a Ramon y Cajal fellowship (RyC-201209975), from the Spanish research project AYA2017-89384-P, and from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). A.P. wishes to acknowledge support by the European Research Council via the ERC consolidating grant #773062 (acronym O.M.J.). A.C. acknowledges the support of NASA MIRO grant NNX15AP95A. Based on observations made with the NASA/ESA Hubble Space Telescope (programme GO 14353, PI Fruchter), obtained through the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Support for this work was also provided by the National Aeronautics and Space Administration through Chandra Award Number 17500753, PI Fruchter, issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester as well as observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma. Development of CIRCE was supported by the University of Florida and the National Science Foundation (grant AST-0352664), in collaboration with IUCAA. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Development of the BOXFIT code was supported in part by NASA through grant NNX10AF62G issued through the Astrophysics Theory Program and by the NSF through grant AST1009863. Simulations for BOXFIT version 2 have been carried out in part on the computing facilities of the Computational Center for Particle and Astrophysics (C2PAP) of the research cooperation "Excellence Cluster Universe" in Garching, Germany.

Energy injection into the external shock system that generates the afterglow to a gamma-ray burst (GRB) can result in a re-brightening of the emission. Here we investigate the off-axis view of a re-brightened refreshed shock afterglow. We find that the afterglow light-curve, when viewed from outside of the jet opening angle, could be characterised by a slow rise, or long-plateau, with a maximum flux determined by the total system energy. Using the broadband afterglow data for GRB170817A, associated with the gravitational wave detected binary neutron star merger GW170817, we show that a refreshed shock model with a simple top-hat jet can reproduce the observed afterglow features. We consider two particular refreshed shock models: a single episode of energy injection; and a period of continuous energy injection. The best fit model parameters give a jet opening angle, for our first or second model of $\theta_j=5.2^{+1.1}_{-0.6}~$or$~6.3^{+1.7}_{-1.1}$ deg, an inclination to the line of sight $\iota=16.0^{+3.4}_{-1.1}~$or$~17.8^{+4.5}_{-2.9}$ deg, an initial isotropic equivalent kinetic energy $E_1 = (0.3^{+3.5}_{-0.3}~$or$~0.5^{+6.7}_{-0.2})\times10^{52}$erg and a total/final, refreshed shock energy $E_{\rm total}=(0.42^{+5.6}_{-0.4}~$or$~1.26^{+18.2}_{-0.7})\times10^{53}$erg. The first model fitting prefers an initial bulk Lorentz factor $\Gamma_{0,1}, Comment: 14 pages, 6 figures - Version accepted for publication in ApJ. Analysis now includes two refreshed shock models and expanded discussion

In this work we present spectra of all γ-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31/03/2017. In total, we have obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimise biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneously selected sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We have constrained the fraction of dark bursts to be, JPUF, BMJ and DX acknowledge support from the ERC-StG grant EGGS-278202. The Dark Cosmology Centre was funded by the Danish National Research Foundation. This work was supported by a VILLUM FONDEN Investigator grant to JH (project number 16599). TK acknowledges support by the European Commission under the Marie Curie Intra-European Fellowship Programme in FP7. LK and JJ acknowledges support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy. KEH and PJ acknowledge support by a Project Grant (162948-051) from The Icelandic Research Fund. AG acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1-0031 and project grant No. J1-8136). CT acknowledges support from a Spanish National Research Grant of Excellence under project AYA 2014-58381-P and funding associated to a Ramon y Cajal fellowship under grant number RyC-2012-09984. AdUP acknowledges support from a Ramon y Cajal fellowship, a BBVA Foundation Grant for Researchers and Cultural Creators, and the Spanish Ministry of Economy and Competitiveness through project AYA2014-58381-P. ZC acknowledges support from the Spanish research project AYA 2014-58381-P and support from Juan de la Cierva Incorporacion fellowships IJCI-2014-21669. DAK acknowledges support from the Spanish research project AYA 2014-58381-P and support from Juan de la Cierva Incorporacion fellowships IJCI-2015-26153. RSR acknowledges AdUP's BBVA Foundation Grant for Researchers and Cultural Creators and support from ASI (Italian Space Agency) through the Contract n. 2015-046-R.0 and from European Union Horizon 2020 Programme under the AHEAD project (grant agreement n. 654215). GL is supported by a research grant (19054) from VILLUM FONDEN. SDV acknowledges the support of the French National Research Agency (ANR) under contract ANR-16-CE31-0003 BEaPro DM acknowledges support from the Instrument Center for Danish Astrophysics (IDA).

We present X-ray and optical observations of the short duration gamma-ray burst GRB 071227 and its host at $z=0.381$, obtained using \textit{Swift}, Gemini South and the Very Large Telescope. We identify a short-lived and moderately bright optical transient, with flux significantly in excess of that expected from a simple extrapolation of the X-ray spectrum at 0.2-0.3 days after burst. We fit the SED with afterglow models allowing for high extinction and thermal emission models that approximate a kilonova to assess the excess' origins. While some kilonova contribution is plausible, it is not favoured due to the low temperature and high luminosity required, implying superluminal expansion and a large ejecta mass of $\sim 0.1$ M$_{\odot}$. We find, instead, that the transient is broadly consistent with power-law spectra with additional dust extinction of $E(B-V)\sim0.4$ mag, although a possibly thermal excess remains in the \textit{z}-band. We investigate the host, a spiral galaxy with an edge-on orientation, resolving its spectrum along its major axis to construct the galaxy rotation curve and analyse the star formation and chemical properties. The integrated host emission shows evidence for high extinction, consistent with the afterglow findings. The metallicity and extinction are consistent with previous studies of this host and indicate the galaxy is a typical, but dusty, late-type SGRB host., Comment: 16 pages, 13 figures, accepted to MNRAS

Whether stars could have driven the reionization of the intergalactic medium depends critically on the proportion of ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst (GRB) afterglows can be used to estimate the opacity to extreme ultraviolet (EUV) radiation along the lines-of-sight to the bursts. Assuming that long-duration GRBs trace the locations of the massive stars dominating EUV production, the average escape fraction of ionizing radiation can be calculated independently of galaxy size or luminosity. Here we present a compilation of H i column density (N HI ) measures for 140 GRBs in the range 1.6 < z < 6.7. Although the sample is heterogeneous, in terms of spectral resolution and signal-to-noise ratio, fits to the Ly α absorption line provide robust constraints on N HI , even for spectra of insufficient quality for other purposes. Thus we establish an escape fraction at the Lyman limit of (f esc ) ≈ 0.005, with a 98 per cent confidence upper limit of (f esc ) ≈ 0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget at z < 5. At higher redshifts firm conclusions are limited by the small size of the GRB sample (7/140), but any decline in average H i column density seems to be modest. We also find no significant correlation of N HI with galaxy UV luminosity or host stellar mass. We discuss in some detail potential biases and argue that, while not negligible, systematic errors in f esc are unlikely to be more than a factor ~2 in either direction, and so would not affect the primary conclusions. Given that many GRB hosts are low-metallicity dwarf galaxies with high specific star-formation rates, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the Universe. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society., JJ acknowledges support from Nederlandse Onderzoekschool Voor Astronomie(NOVA) and The Netherlands Organisation for Scientific Research (NWO) - The Sao Paulo Research Foundation (FAPESP) grant for advanced instrumentation in astronomy. KEH acknowledges support by a Project Grant (162948-051) from The Icelandic Research Fund. DAK acknowledges support from the Spanish research project AYA2014-58381-P and Juan de la Cierva Incorporacion IJCI-2015-26153. AJL and ERS acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/L000733/1. NRT and KW acknowledge Science and Technology Facilities Council (STFC) consolidated grant ST/N000757/1. AC acknowledges National Aeronautics and Space Administration (NASA) grant NNX15AP95A. AdUP acknowledges support from a Ramon y Cajal fellowship (RyC-2012-09975), a 2016 Banco Bilbao Vizcaya Argentaria (BBVA) Foundation Grant for Researchers and Cultural Creators, and from the Spanish research project AYA 2014-58381-P. RC acknowledges partial support from National Aeronautics and Space Administration (NASA) Swift grant NNX16AB04G.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246).

We present a study of 21 dark gamma-ray burst (GRB) host galaxies, predominantly using X-ray afterglows obtained with the Chandra X-Ray Observatory (CXO) to precisely locate the burst in deep Hubble Space Telescope (HST) imaging of the burst region. The host galaxies are well-detected in F160W in all but one case and in F606W imaging in approx 60 per cent of cases. We measure magnitudes and perform a morphological analysis of each galaxy. The asymmetry, concentration and ellipticity of the dark burst hosts are compared against the host galaxies of optically bright GRBs. In agreement with other studies, we find that dark GRB hosts are redder and more luminous than the bulk of the GRB host population. The distribution of projected spatial offsets for dark GRBs from their host galaxy centroids is comparable to that of optically-bright bursts. The dark GRB hosts are physically larger, more massive and redder, but are morphologically similar to the hosts of bright GRBs in terms of concentration and asymmetry. Our analysis constrains the fraction of high redshift (z greater than 5) GRBs in the sample to approx 14 per cent, implying an upper limit for the whole long-GRB population of less than 4.4 per cent. If dust is the primary cause of afterglow darkening amongst dark GRBs, the measured extinction may require a clumpy dust component in order to explain the observed offset and ellipticity distributions., Comment: Accepted for publication in MNRAS

Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts. In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon. Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst. Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star. This cocoon rapidly becomes transparent and is outshone by the supernova emission, which starts to dominate the emission three days after the burst. © 2019, Springer Nature Limited., L.I. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2016-30940. L.I., A.d.U.P., C.C.T. and D.A.K. acknowledge support from the Spanish research project AYA2017-89384-P. A.d.U.P. acknowledges support from funding associated with Ramon y Cajal fellowship RyC-2012-09975. C.C.T. acknowledges support from funding associated with Ramon y Cajal fellowship RyC-2012-09984. D.A.K. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. K.M. acknowledges support from JSPS Kakenhi grants (18H05223, 18H04585 and 17H02864). S. Schmidl acknowledges support from grant DFG Klose 766/16-3 and discussions with S. Klose. R.L.C.S. acknowledges funding from STFC. M.J.M. acknowledges the support of the National Science Centre, Poland, through POLONEZ grant 2015/19/P/ST9/04010; this project has received funding from the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement number 665778. R.S.-R. acknowledges support from ASI (Italian Space Agency) through contract number 2015-046-R.0 and from the European Union's Horizon 2020 programme under the AHEAD project (grant agreement number 654215). The Cosmic Dawn Center is funded by the DNRF. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). G.L. was supported by a research grant from VILLUM FONDEN (project number 19054). K.E.H. acknowledges support by a Project Grant (162948-051) from The Icelandic Research fund. J.J. and L.K. acknowledge support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy.

We present observations of the optical afterglow of GRB\,170817A, made by the {\it Hubble Space Telescope}, between February and August 2018, up to one year after the neutron star merger, GW170817. The afterglow shows a rapid decline beyond $170$~days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for `failed-jet' scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with Lorentz factor $\Gamma\gtrsim100$, forms a narrow core ($\sim5^\circ$) and is surrounded by a wider angular component that extends to $\sim15^\circ$, which is itself relativistic ($\Gamma\gtrsim5$). For a two-component model of this structure, the late-time optical decline, where $F \propto t^{-\alpha}$, is $\alpha=2.20\pm0.18$, and for a Gaussian structure the decline is $\alpha=2.45\pm0.23$. We find the Gaussian model to be consistent with both the early $\sim10$ days and late $\gtrsim290$ days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution and its continued decline indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster, with a luminosity $\lesssim 4000 L_\odot~(M_{\rm F606W}\gtrsim-4.3)$., Comment: 8 pages, 3 figures, accepted for publication in ApJL

The death of massive stars, manifested as gamma-ray bursts and core-collapse supernovae, critically influence how the universe formed and evolves. Despite their fundamental importance, our understanding of these enigmatic objects is severely limited. We have performed a concept study of an Astrophysical Transient Observatory (ATO) that will rapidly facilitate an expansion of our understanding of these objects. ATO combines a very wide-field X-ray telescope, a near-infrared telescope, a multi-mode ultraviolet instrument, and a rapidly slewing spacecraft to realize two primary goals: (1) characterize the highest-redshift massive stars and their environments, and (2) constrain the poorly understood explosion mechanism of massive stars. The goals are met by observing the first massive stars to explode as gamma-ray bursts and to probe their environments, and by observing the shock breakout of core-collapse supernovae to measure the outer envelope parameters of massive stars. Additionally, ATO will observe the shock breakout of Type Ia supernovae and their shock interaction with a companion, electromagnetic counterparts to gravitational wave sources, kilonovae, tidal disruption events, cataclysmic variables, X-ray transients, flares from exoplanet host stars, and the escape of ionizing radiation from star-forming galaxies. A description of the ATO instruments, the mission simulation, and technology readiness level is provided.

The extinction profiles in gamma-ray burst (GRB) afterglow spectral energy distributions (SEDs) are usually described by the small magellanic cloud (SMC)-type extinction curve. In different empirical extinction laws, the total-to-selective extinction, R, is an important quantity because of its relation to dust grain sizes and compositions. We here analyse a sample of 17 GRBs (0.34 < z < 7.84) where the ultraviolet to near-infrared spectroscopic observations are available through the VLT/X-shooter instrument, giving us an opportunity to fit individual extinction curves of GRBs for the first time. Our sample is compiled on the basis of the availability of multiband photometry around the X-shooter observations. The X-shooter data are combined with the Swift X-ray data and a single or broken power law together with a parametric extinction law is used to model the individual SEDs. We find 10 cases with significant dust, where the derived extinction, A, ranges from 0.1-1.0mag. In four of those, the inferred extinction curves are consistent with the SMC curve. The GRB individual extinction curves have a flat R distribution with an optimal weighted combined value of R = 2.61 ± 0.08 (for seven broad coverage cases). The 'average GRB extinction curve' is similar to, but slightly steeper than the typical SMC, and consistent with the SMCBar extinction curve at~95 per cent confidence level. The resultant steeper extinction curves imply populations of small grains, where large dust grains may be destroyed due to GRB activity. Another possibility could be that young age and/or lower metallicities of GRBs environments are responsible for the steeper curves.© 2018 The Author(s)., The X-ray data for this work is obtained from the UK Swift Science Data Center at the University of Leicester. KEH acknowledges support by a Project Grant (162948-051) from The Icelandic Research Fund. AdUP acknowledges support from the Spanish research project AYA 2014-58381-P, from a RyC fellowship and from a BBVA Foundation grant for Researchers and Cultural Creators. JJ acknowledges support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy.

We report the detection of the radio afterglow of a long gamma-ray burst (GRB) 111005A at 5-345 GHz, including the very long baseline interferometry observations with the positional error of 0.2 mas. The afterglow position is coincident with the disk of a galaxy ESO 580-49 at z= 0.01326 (~1" from its center), which makes GRB 111005A the second closest GRB known to date, after GRB 980425. The radio afterglow of GRB 111005A was an order of magnitude less luminous than those of local low-luminosity GRBs, and obviously than those of cosmological GRBs. The radio flux was approximately constant and then experienced an unusually rapid decay a month after the GRB explosion. Similarly to only two other GRBs, we did not find the associated supernovae (SN), despite deep near- and mid-infrared observations 1-9 days after the GRB explosion, reaching ~20 times fainter than other SNe associated with GRBs. Moreover, we measured twice solar metallicity for the GRB location. The low gamma-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a different rare class of GRBs than typical core-collapse events. We modelled the spectral energy distribution of the GRB 111005A host finding that it is a dwarf, moderately star-forming galaxy, similar to the host of GRB 980425. The existence of two local GRBs in such galaxies is still consistent with the hypothesis that the GRB rate is proportional to the cosmic star formation rate (SFR) density, but suggests that the GRB rate is biased towards low SFRs. Using the far-infrared detection of ESO 580-49, we conclude that the hosts of both GRBs 111005A and 980425 exhibit lower dust content than what would be expected from their stellar masses and optical colours., Comment: Accepted by A&A. 17 pages, 16 figures, 5 tables

We identify and explore the properties of an infrared-bright gamma-ray burst (GRB) host population. Candidate hosts are selected by coincidence with sources in WISE, with matching to random coordinates and a false alarm probability analysis showing that the contamination fraction is approx 0.5. This methodology has already identified the host galaxy of GRB 080517. We combine survey photometry from Pan-STARRS, SDSS, APASS, 2MASS, GALEX and WISE with our own WHT/ACAM and VLT/X- shooter observations to classify the candidates and identify interlopers. Galaxy SED fitting is performed using MAGPHYS, in addition to stellar template fitting, yielding 13 possible IR-bright hosts. A further 7 candidates are identified from previously published work. We report a candidate host for GRB 061002, previously unidentified as such. The remainder of the galaxies have already been noted as potential hosts. Comparing the IR-bright population properties including redshift z, stellar mass M*, star formation rate SFR and V-band attenuation Av to GRB host catalogues in the literature, we find that the infrared-bright population is biased toward low z, high M* and high Av. This naturally arises from their initial selection - local and dusty galaxies are more likely to have the required IR flux to be detected in WISE. We conclude that while IR-bright GRB hosts are not a physically distinct class, they are useful for constraining existing GRB host populations, particularly for long GRBs., Comment: Accepted for publication in MNRAS

It is notoriously difficult to localize short γ-ray bursts (sGRBs) and their hosts to measure their redshifts. These measurements, however, are critical for constraining the nature of sGRB progenitors, their redshift distribution, and the r-process element enrichment history of the universe. Here we present spectroscopy of the host galaxy of GRB 111117A and measure its redshift to be z = 2.211. This makes GRB 111117A the most distant high-confidence short duration GRB detected to date. Our spectroscopic redshift supersedes a lower, previously estimated photometric redshift value for this burst. We use the spectroscopic redshift, as well as new imaging data to constrain the nature of the host galaxy and the physical parameters of the GRB. The rest-frame X-ray derived hydrogen column density, for example, is the highest compared to a complete sample of sGRBs and seems to follow the evolution with redshift as traced by the hosts of long GRBs. From the detection of Lyα emission in the spectrum, we are able to constrain the escape fraction of Lyα in the host. The host lies in the brighter end of the expected sGRB host brightness distribution at z = 2.211, and is actively forming stars. Using the observed sGRB host luminosity distribution, we find that between 43% and 71% of all Swift-detected sGRBs have hosts that are too faint at z ∼ 2 to allow for a secure redshift determination. This implies that the measured sGRB redshift distribution could be incomplete at high redshift. The high z of GRB 111117A is evidence against a lognormal delay-time model for sGRBs through the predicted redshift distribution of sGRBs, which is very sensitive to high-z sGRBs. From the age of the universe at the time of GRB explosion, an initial neutron star (NS) separation of a < 3.1 R is required in the case where the progenitor system is a circular pair of inspiralling NSs. This constraint excludes some of the longest sGRB formation channels for this burst.© 2018 ESO., We thank the anonymous referee for the constructive report. We thank Jens Hjorth and Lise Christensen for useful discussions regarding the interpretation of this event. We thank Mathieu Puech for testing the possible contribution from an older stellar population in the SED. We thank Peter Laursen for fruitful discussions regarding the Ly alpha escape fraction. TK acknowledges support through the Sofja Kovalevskaja Award to P. Schady. SDV is supported by the French National Research Agency (ANR) under contract ANR-16-CE31-0003 BEaPro. PDA and SCo acknowledge support from ASI grant I/004/11/3. JJ acknowledges support from NOVA and a NWO-FAPESP grant for advanced instrumentation in astronomy. NRT and KW acknowledge support from STFC Consolidated Grant ST/N000757/1. CT acknowledges support from a Spanish National Research Grant of Excellence under project AYA 2014-58381-P and funding associated with a Ramon y Cajal fellowship under grant number RyC-2012-09984. AdUP acknowledges support from a Ramon y Cajal fellowship, a BBVA Foundation Grant for Researchers and Cultural Creators, and the Spanish Ministry of Economy and Competitiveness through project AYA2014-58381-P. ZC acknowledges support from the Spanish research project AYA 2014-58381-P and support from Juan de la Cierva Incorporacion fellowships IJCI-2014-21669. RSR acknowledges AdUP's BBVA Foundation Grant for Researchers and Cultural Creators and support from the Italian Space Agency (ASI) through Contract n. 2015-046-R.0 and from the European Union Horizon 2020 Programme under the AHEAD project (grant agreement n. 654215). This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013). The analysis and plotting was achieved using the Python-based packages Matplotlib (Hunter 2007), Numpy, and Scipy (van der Walt et al. 2011), along with other community-developed packages. This work made use of observations obtained with the Italian 3.6m Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Based on data from the GTC Archive at CAB (INTA-CSIC) and on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnologia e Innovacion Productiva (Argentina), and Ministerio da Ciencia, Tecnologia e Inovacao (Brazil).

Recent observations with Swift have begun to uncover $\gamma$-ray transients whose total energies are comparable to those of gamma-ray bursts (GRB), but have a duration an order of magnitude or more longer than the bulk of the GRB population. Some are suggested to form a new population of ultra-long GRBs, with a mean duration around $10^4$s, while a further population with $\gamma-$ray durations $>10^5$ s may represent manifestations of relativistic outflows from stars shredded around massive black holes in tidal disruption flares (TDFs). Here I review the observations of these new classes of events, discuss progress towards identifying their progenitors and suggest how new observations may both hone our understanding of the outbursts, and allow them to be used as probes, that offer both complementary and additional tools to GRBs., Comment: 14 pages, 9 figures. To appear in the Journal of High Energy Astrophysics, special issue "Swift: Ten Years of Discovery"

GRB 051103 is considered to be a candidate soft gamma repeater (SGR) extragalactic giant magnetar flare by virtue of its proximity on the sky to M81/M82, as well as its time history, localization, and energy spectrum. We have derived a refined interplanetary network localization for this burst which reduces the size of the error box by over a factor of two. We examine its time history for evidence of a periodic component, which would be one signature of an SGR giant flare, and conclude that this component is neither detected nor detectable under reasonable assumptions. We analyze the time-resolved energy spectra of this event with improved time- and energy resolution, and conclude that although the spectrum is very hard, its temporal evolution at late times cannot be determined, which further complicates the giant flare association. We also present new optical observations reaching limiting magnitudes of R > 24.5, about 4 magnitudes deeper than previously reported. In tandem with serendipitous observations of M81 taken immediately before and one month after the burst, these place strong constraints on any rapidly variable sources in the region of the refined error ellipse proximate to M81. We do not find any convincing afterglow candidates from either background galaxies or sources in M81, although within the refined error region we do locate two UV bright star forming regions which may host SGRs. A supernova remnant (SNR) within the error ellipse could provide further support for an SGR giant flare association, but we were unable to identify any SNR within the error ellipse. These data still do not allow strong constraints on the nature of the GRB 051103 progenitor, and suggest that candidate extragalactic SGR giant flares will be difficult, although not impossible, to confirm.

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides., version accepted for publication in Nature. Some minor changes are expected with respect to the journal version

We present the results of a Hubble Space Telescope WFC3/F160W SNAPSHOT sur- vey of the host galaxies of 39 long-duration gamma-ray bursts (LGRBs) at z < 3. We have non-detections of hosts at the locations of 4 bursts. Sufficient accuracy to as- trometrically align optical afterglow images and determine the location of the LGRB within its host was possible for 31/35 detected hosts. In agreement with other work, we find the luminosity distribution of LGRB hosts is significantly fainter than that of a star formation rate-weighted field galaxy sample over the same redshift range, indicating LGRBs are not unbiasedly tracing the star formation rate. Morphologi- cally, the sample of LGRB hosts are dominated by spiral-like or irregular galaxies. We find evidence for evolution of the population of LGRB hosts towards lower-luminosity, higher concentrated hosts at lower redshifts. Their half-light radii are consistent with other LGRB host samples where measurements were made on rest-frame UV obser- vations. In agreement with recent work, we find their 80 per cent enclosed flux radii distribution to be more extended than previously thought, making them intermedi- ate between core-collapse supernova (CCSN) and super-luminous supernova (SLSN) hosts. The galactocentric projected-offset distribution confirms LGRBs as centrally concentrated, much more so than CCSNe and similar to SLSNe. LGRBs are strongly biased towards the brighter regions in their host light distributions, regardless of their offset. We find a correlation between the luminosity of the LGRB explosion site and the intrinsic column density, N_H , towards the burst., 26 pages, accepted to MNRAS

We present a comprehensive study of the relations between gas kinematics, metallicity, and stellar mass in a sample of 82 GRB-selected galaxies using absorption and emission methods. We find the velocity widths of both emission and absorption profiles to be a proxy of stellar mass. We also investigate the velocity-metallicity correlation and its evolution with redshift and find the correlation derived from emission lines to have a significantly smaller scatter compared to that found using absorption lines. Using 33 GRB hosts with measured stellar mass and metallicitiy, we study the mass-metallicity relation for GRB host galaxies in a stellar mass range of $10^{8.2} M_{\odot}$ to $10^{11.1} M_{\odot}$ and a redshift range of $ z\sim 0.3-3.4$. The GRB-selected galaxies appear to track the mass-metallicity relation of star forming galaxies but with an offset of 0.15 towards lower metallicities. This offset is comparable with the average error-bar on the metallicity measurements of the GRB sample and also the scatter on the MZ relation of the general population. It is hard to decide whether this relatively small offset is due to systematic effects or the intrinsic nature of GRB hosts. We also investigate the possibility of using absorption-line metallicity measurements of GRB hosts to study the mass-metallicity relation at high redshifts. Our analysis shows that the metallicity measurements from absorption methods can significantly differ from emission metallicities and assuming identical measurements from the two methods may result in erroneous conclusions., Accepted for publication in MNRAS

Gamma-ray burst (GRB) 111215A was bright at X-ray and radio frequencies, but not detected in the optical or near-infrared (nIR) down to deep limits. We have observed the GRB afterglow with the Westerbork Synthesis Radio Telescope and Arcminute Microkelvin Imager at radio frequencies, with the William Herschel Telescope and Nordic Optical Telescope in the nIR/optical, and with the Chandra X-ray Observatory. We have combined our data with the Swift X-Ray Telescope monitoring, and radio and millimeter observations from the literature to perform broadband modeling, and determined the macro- and microphysical parameters of the GRB blast wave. By combining the broadband modeling results with our nIR upper limits we have put constraints on the extinction in the host galaxy. This is consistent with the optical extinction we have derived from the excess X-ray absorption, and higher than in other dark bursts for which similar modeling work has been performed. We also present deep imaging of the host galaxy with the Keck I telescope, Spitzer Space Telescope, and Hubble Space Telescope (HST), which resulted in a well-constrained photometric redshift, giving credence to the tentative spectroscopic redshift we obtained with the Keck II telescope, and estimates for the stellar mass and star formation rate of the host. Finally, our high resolution HST images of the host galaxy show that the GRB afterglow position is offset from the brightest regions of the host galaxy, in contrast to studies of optically bright GRBs., Comment: 11 pages, 4 figures, 4 tables; accepted for publication in MNRAS

Gamma-ray bursts (GRBs) are most probably powered by collimated relativistic outflows (jets) from accreting black holes at cosmological distances. Bright afterglows are produced when the outflow collides with the ambient medium. Afterglow polarization directly probes the magnetic properties of the jet, when measured minutes after the burst, and the geometric properties of the jet and the ambient medium when measured hours to days after the burst. High values of optical polarization detected minutes after burst in GRB 120308A indicate the presence of large-scale ordered magnetic fields originating from the central engine (the power source of the GRB). Theoretical models predict low degrees of linear polarization and negligable circular polarization at late times, when the energy in the original ejecta is quickly transferred to the ambient medium and propagates farther into the medium as a blastwave. Here we report the detection of circularly polarized optical light in the afterglow of GRB 121024A, measured 0.15 days after the burst. We show that the circular polarization is intrinsic to the afterglow and unlikely to be produced by dust scattering or plasma propagation effects. A possible explanation is to invoke anisotropic (rather than the commonly assumed isotropic) electron pitch angle distributions, and we suggest that new models are required to produce the complex microphysics of realistic shocks in relativistic jets., Published as Wiersema et al. 2014, Nature 509, 201-204. This is the version prior to final editing; please see official published version for the final version and higher quality images

We report our identification of the optical afterglow and host galaxy of the short-duration gamma-ray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0.162, making it one of the lowest redshift short-duration gamma-ray bursts (sGRBs) identified by Swift. Our intensive follow-up campaign using a range of ground-based facilities as well as Hubble Space Telescope, XMM-Newton, and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light curve at X-ray frequencies reveals a narrow jet, deg, that is refreshed at >1 day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetized shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass M = (1.0 ±0.6) ×10 M and a secular (post-merger) ejecta mass with M = (1.0 ±0.6) ×10 M , consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared data set provides the best-sampled kilonova light curve without a gravitational wave trigger to date.© 2019. The American Astronomical Society. All rights reserved., Partly based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma; and with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos (program 51-504); and with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos (program A32TAC_5). The development of CIRCE at GTC was supported by the University of Florida and the National Science Foundation (grant AST-0352664), in collaboration with IUCAA. Based on data from the GTC Public Archive at CAB (INTA-CSIC). N.R.T., A.J.L., K.W., and B.G. have received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246, TEDE, PI Levan). A.J.L. and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T. and G.P.L. acknowledge support from STFC via grant ST/N000757/1. E.P. acknowledges support from grant ASI/INAF I/088/06/0. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). D.B.M. acknowledges support from the Instrument center for Danish astrophysics (IDA). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. A.C. acknowledges support from the National Science Foundation CAREER award #1455090. A.d.U.P., C.C.T., Z.C., L.I., and D.A.K. acknowledge support from the Spanish research projects AYA2014-58381-P and AYA2017-89384-P, from the State Agency for Research of the Spanish MCIU through the >Center of Excellence Severo Ochoa> award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). A.d.U.P. and C.C.T. acknowledge support from Ramon y Cajal fellowships (RyC-2012-09975, and RyC-2012-09984). Z.C., L.I., and D.A.K. acknowledge support from Juan de la Cierva Incorporacion fellowships (JdCI-2014-21669, IJCI-2016-30940, and IJCI-2015-26153). P.A.E. and K.L.P. acknowledge support from the UK Space Agency. K.E.H. and P.J. acknowledge support by a Project grant (162948-051) from The Icelandic Research Fund. S.R. has been supported by the Swedish Research Council (VR) under grant No. 2016-03657_3, by the Swedish National Space Board under grant No. Dnr 107/16 and by the research environment grant >Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)> funded by the Swedish Research council (VR) under Dnr 2016-06012.

Abundances of galaxies at redshifts z > 4 are difficult to obtain from damped Ly {\alpha} (DLA) systems in the sightlines of quasars (QSOs) due to the Ly {\alpha} forest blanketing and the low number of high-redshift quasars detected. Gamma-ray bursts (GRBs) with their higher luminosity are well suited to study galaxies out to the formation of the first stars at z > 10. Its large wavelength coverage makes the X-shooter spectrograph an excellent tool to study the interstellar medium (ISM) of high redshift galaxies, in particular if the redshift is not known beforehand. Here we determine the properties of a GRB host at z = 4.66723 from a number of resonant low- and high ionization and fine-structure absorption lines. This is one of the highest redshifts where a detailed analysis with medium-resolution data has been possible. We detect one intervening system at z = 2.18. The velocity components of the absorption lines are fitted with Voigt-profiles and we determine a metallicity of [M/H] = -1.0 \pm 0.1 using S. The absorption lines show a complicated kinematic structure which could point to a merger in progress. Si II* together with the restframe UV energy release determined from GROND data gives us the distance of 0.3 to 1 kpc of the absorbing material from the GRB. We measure a low extinction of AV = 0.24 \pm 0.06 mag using X-ray spectral information and the flux calibrated X-shooter spectrum. GRB-DLAs have a shallower evolution of metallicity with redshift than QSO absorbers and no evolution in HI column density or ionization fraction. GRB hosts at high redshift might continue the trend towards lower metallicities in the LZ-relation with redshift, but the sample is still too small to draw a definite conclusion. While the detection of GRBs at z > 4 with current satellites is still difficult, they are very important for our understanding of the early epochs of star- and galaxy-formation., Comment: 16 pages, 15 figures. Resubmitted to A&A after referee comments

We report on observations of GRB 080503, a short gamma-ray burst with very bright extended emission (about 30 times the gamma-ray fluence of the initial spike) in conjunction with a thorough comparison to other short Swift events. In spite of the prompt-emission brightness, however, the optical counterpart is extraordinarily faint, never exceeding 25 mag in deep observations starting at ~1 hr after the BAT trigger. The optical brightness peaks at ~1 day and then falls sharply in a manner similar to the predictions of Li & Paczynski (1998) for supernova-like emission following compact-binary mergers. However, a shallow spectral index and similar evolution in X-rays inferred from Chandra observations are more consistent with an afterglow interpretation. The extreme faintness of this probable afterglow relative to the bright gamma-ray emission argues for a very low-density medium surrounding the burst (a "naked" GRB), consistent with the lack of a coincident host galaxy down to 28.5 mag in deep HST imaging. Our observations reinforce the notion that short GRBs generally occur outside regions of active star formation, but demonstrate that in some cases the luminosity of the extended prompt emission can greatly exceed that of the short spike, which may constrain theoretical interpretation of this class of events. Because most previous BAT short bursts without observed extended emission are too faint for this signature to have been detectable even if it were present at typical level, conclusions based solely on the observed presence or absence of extended emission in the existing Swift sample are premature. (abridged), 16 pages, 11 figures; accepted for publication in ApJ

Gamma-ray bursts are powerful probes of the early universe, but locating and identifying very distant GRBs remains challenging. We report here the discovery of the K-band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high redshift burst (z>11). However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely z, Submitted to MNRAS (revised in light of referee's comments)

We present rest-frame NIR luminosities and stellar masses for a large and uniformly-selected population of GRB host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find strong evolution in the host luminosity distribution between z~0.5 (median absolute NIR AB magnitude ~ -18.5, corresponding to M* ~ 3x10^8 M_sun and z~1.5), but negligible variation between z~1.5 and z~5 (median magnitude ~ -21.2, corresponding to M* ~ 5x10^9 M_sun). Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high-redshift while low-mass star-forming galaxies retain little dust in their ISM. Comparing our luminosity distributions to field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star-formation is constant in galaxies with gas-phase metallicity below approximately the Solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously-reported "excess" in the GRB rate beyond z>2; metals stifle GRB production in most galaxies at z, Published in ApJ

[Abridged] A number of phenomena have been observed in GRB afterglows that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well calibrated dataset of a typical afterglow to serve as a benchmark system for modelling afterglow polarisation behaviour. In particular, our dataset of the afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high quality optical and near-infrared broadband light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarisation varies between 0 and 3%, with both long and short time scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarised component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R and Ks band polarimetry shows that dust induced polarisation in the host galaxy is likely negligible.

We consider a popular model for long-duration gamma-ray bursts, in which the progenitor star, a stripped helium core, is spun up by tidal interactions with a black- hole companion in a compact binary. We perform population synthesis calculations to produce a representative sample of such binaries, and model the effect that the companion has on material that falls back on to the newly-formed black hole. Taking the results of hydrodynamic models of black-hole formation by fallback as our starting point, we show that the companion has two main effects on the fallback process. First, a break forms in the accretion curve at around 10 000 s. Secondly, subsequent to the break, we expect to see a flare of total energy around 0.1 foe. We predict that the break time is set largely by the semi-major axis of the binary at the time of explosion, and that this correlates negatively with the flare energy. Although comparison with observations is non-trivial, we show that our predicted break times are comparable to those found in the X-ray light curves of canonical long-duration gamma-ray bursts. Similarly, the flare properties that we predict are consistent with the late-time flares observed in a sub-sample of bursts.

We present deep infrared (Ks band) imaging polarimetry and radio (1.4 and 4.8 GHz) polarimetry of the enigmatic transient Swift J164449.3+573451. This source appears to be a short lived jet phenomenon in a galaxy at redshift z = 0.354, activated by a sudden mass accretion onto the central massive black hole, possibly caused by the tidal disruption of a star. We aim to find evidence for this scenario through linear polarimetry, as linear polarisation is a sensitive probe of jet physics, source geometry and the various mechanisms giving rise to the observed radiation. We find a formal Ks band polarisation measurement of P_lin = 7.4 +/- 3.5 % (including systematic errors). Our radio observations show continuing brightening of the source, which allows sensitive searches for linear polarisation as a function of time. We find no evidence of linear polarisation at radio wavelengths of 1.4 GHz and 4.8 GHz at any epoch, with the most sensitive 3 sigma limits as deep as 2.1%. These upper limits are in agreement with expectations from scenarios in which the radio emission is produced by the interaction of a relativistic jet with a dense circumsource medium. We further demonstrate how the polarisation properties can be used to derive properties of the jet in Swift J164449.3+573451, exploiting the similarities between this source and the afterglows of gamma-ray bursts.

GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the GRB environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the NIR-to-X-ray spectral energy distribution (SED) and studying the high signal-to-noise VLT/FORS afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the UV, low equivalent hydrogen column density and little reddening caused by a LMC- or SMC-type extinction curve. From the weak MgII absorption at z=1.5477 in the spectrum, we derived logN(MgII)=12.96+0.13-0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely an LLS with a 0.03Z\odot

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionisation state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z=2.609, whose optical spectrum indicates an almost fully ionised medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Ly alpha absorption line. Photoionisation modeling shows that we are probing material from the vicinity of the burst (~80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies.

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them to the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies. One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is mis-identified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters.

We present ground-based and HST optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z = 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broadband spectral energy distributions (SEDs) of the OT resemble those of local type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB-supernova, and SN1994I, a typical type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly-available SWIFT -XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 days. We then compare the rest-frame, peak V -band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe.

We report the Swift discovery of nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low redshift host galaxy and associated supernova. We derive the redshift of the event to be z = 0.0591 +/- 0.0001 and provide accurate astrometry for the GRB-SN. We study the extremely unusual prompt emission with time-resolved gamma-ray to X-ray spectroscopy, and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini South, VLT and HST observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB--SN connection can be gleaned.

Following the faint gamma-ray burst, GRB 170817A, coincident with a gravitational wave-detected binary neutron star merger at d &sim, 40 Mpc, we consider the constraints on a local population of faint short duration GRBs (defined here broadly as T 90 &lt, 4 s). We review proposed low-redshift short-GRBs and consider statistical limits on a d ≲ 200 Mpc population using Swift/Burst Alert Telescope (BAT), Fermi/Gamma-ray Burst Monitor (GBM), and Compton Gamma-Ray Observatory (CGRO) Burst and Transient Source Experiment (BATSE) GRBs. Swift/BAT short-GRBs give an upper limit for the all-sky rate of &lt, 4 y &minus, 1 at d &lt, 200 Mpc, corresponding to &lt, 5% of SGRBs. Cross-correlation of selected CGRO/BATSE and Fermi/GBM GRBs with d &lt, 100 Mpc galaxy positions returns a weaker constraint of ≲ 12 y &minus, 1 . A separate search for correlations due to SGR giant flares in nearby ( d &lt, 11 Mpc) galaxies finds an upper limit of &lt, 3 y &minus, 1 . Our analysis suggests that GRB 170817A-like events are likely to be rare in existing SGRB catalogues. The best candidate for an analogue remains GRB 050906, where the Swift/BAT location was consistent with the galaxy IC 0327 at d &asymp, 132 Mpc. If binary neutron star merger rates are at the high end of current estimates, then our results imply that at most a few percent will be accompanied by detectable gamma-ray flashes in the forthcoming LIGO/Virgo science runs.

We present 5.5- and 9.0-GHz observations of five gamma-ray burst host galaxies at z < 0.5, taken using the Australia Telescope Compact Array. We determine tight constraints on the radio continuum flux of four sources (GRB hosts 060218, 060614, 020819 and 990712) and detect a fifth source, the host of GRB 031203, with a flux density Fν(5.50 GHz) = 216 ± 50 μJy. We discuss the star formation rates of all five sources. Our radio-derived star formation rates (and upper limits) are largely consistent with those derived from optical observations, suggesting either that there is little dust-obscured star formation in these sources, or that their starbursts are too young to have established representative radio continuum emission.