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On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg2), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here, we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, as well as the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, , and an upper limit on the isotropic-equivalent energy of a blast wave E < 4.1 × 1051 erg (assuming typical GRB parameters). [ABSTRACT FROM AUTHOR]

The joint observation of GW170817 and GRB170817A proved that binary neutron star (BNS) mergers are progenitors of short gamma-ray bursts (SGRBs): this established a direct link between the still unsettled SGRB central engine and the outcome of BNS mergers, whose nature depends on the equation of state (EOS) and on the masses of the NSs. We propose a novel method to probe the central engine of SGRBs based on this link. We produce an extended catalogue of BNS mergers by combining recent theoretically predicted BNS merger rate as a function of redshift and the NS mass distribution inferred from measurements of Galactic BNSs. We use this catalogue to predict the number of BNS systems ending as magnetars (stable or supramassive NS) or BHs (formed promptly or after the collapse of a hypermassive NS) for different EOSs, and we compare these outcomes with the observed rate of SGRBs. Despite the uncertainties mainly related to the poor knowledge of the SGRB jet structure, we find that for most EOSs the rate of magnetars produced after BNS mergers is sufficient to power all the SGRBs, while scenarios with only BHs as possible central engine seem to be disfavoured. [ABSTRACT FROM AUTHOR]

We report our observation of the short gamma-ray burst (GRB) GRB170817A, associated to the binary neutron star merger gravitational wave (GW) event GW170817, performed in the X-ray band with XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GWassociations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios. [ABSTRACT FROM AUTHOR]

SVOM (Space-based multi-band astronomical Variable Objects Monitor) is a Sino-French space mission dedicated to the study of Gamma-Ray Bursts (GRBs) in the next decade, capable to detect and localise the GRB emission, and to follow its evolution in the high-energy and X-ray domains, and in the visible and NIR bands. The satellite carries two wide-field high-energy instruments: a coded-mask gamma-ray imager (ECLAIRs; 4-150 keV), and a gamma-ray spectrometer (GRM; 15-5500 keV) that, together, will characterise the GRB prompt emission spectrum over a wide energy range. In this paper we describe the performances of the ECLAIRs and GRM system with different populations of GRBs from existing catalogues, from the classical ones to those with a possible thermal component superimposed to their non-thermal emission. The combination of ECLAIRs and the GRM will provide new insights also on other GRB properties, as for example the spectral characterisation of the subclass of short GRBs showing an extended emission after the initial spike. [ABSTRACT FROM AUTHOR]

From a comprehensive statistical analysis of Swift X-ray light curves of gamma-ray bursts (GRBs) collected from 2004 December to the end of 2010, we found a three-parameter correlation between the isotropic energy emitted in the rest-frame 1-104 keV energy band during the prompt emission (Eγ,iso), the rest-frame peak of the prompt emission energy spectrum (Epk), and the X-ray energy emitted in the rest-frame 0.3-30 keV observed energy band (EX,iso), computed excluding the contribution of the flares. In this paper, we update this correlation with the data collected until 2014 June, expanding the sample size with ~35 per cent more objects, where the number of short GRBs doubled. With this larger sample, we confirm the existence of a universal correlation that connects the prompt and afterglow properties of long and short GRBs. We show that this correlation does not depend on the X-ray light-curve morphology and that further analysis is necessary to firmly exclude possible biases derived by redshift measurements. In addition, we discuss about the behaviour of the peculiar objects as ultra-long GRBs and we propose the existence of an intermediate group between long and short GRBs. Interestingly, two GRBs with uncertain classification fall into this category. Finally, we discuss the physics underlying this correlation, in the contest of the efficiency of conversion of the prompt gamma-ray emission energy into the kinetic energy of the afterglow, the photospheric model, and the cannonball model. [ABSTRACT FROM AUTHOR]

High-redshift gamma-ray bursts (GRBs) offer several advantages when studying the distant Universe, providing unique information about the structure and properties of the galaxies in which they exploded. Spectroscopic identification with large ground-based telescopes has improved our knowledge of this kind of distant events. We present the multi-wavelength analysis of the high-z Swift GRB GRB 140515A (z = 6.327). The best estimate of the neutral hydrogen fraction of the intergalactic medium towards the burst is xHI ≤ 0.002. The spectral absorption lines detected for this event are the weakest lines ever observed in GRB afterglows, suggesting that GRB 140515A exploded in a very low-density environment. Its circum-burst medium is characterised by an average extinction (AV ~ 0.1) that seems to be typical of z ≥ 6 events. The observed multi-band light curves are explained either with a very hard injected spectrum (p = 1.7) or with a multi-component emission (p = 2.1). In the second case a long-lasting central engine activity is needed in order to explain the late time X-ray emission. The possible origin of GRB 140515A in a Pop III (or in a Pop II star with a local environment enriched by Pop III) massive star is unlikely. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are detectable in the γ-ray band if their jets are oriented toward the observer. However, for each GRB with a typical &#952jet, there should be ~2/&#952² jet bursts whose emission cone is oriented elsewhere in space. These off-axis bursts can eventually be detected when, due to the deceleration of their relativistic jets, the beaming angle becomes comparable to the viewing angle. Orphan afterglows (OAs) should outnumber the current population of bursts detected in the γ-ray band even if they have not been conclusively observed so far at any frequency. We compute the expected flux of the population of orphan afterglows in the mm, optical, and X-ray bands through a population synthesis code of GRBs and the standard afterglow emission model. We estimate the detection rate of OAs with ongoing and forthcoming surveys. The average duration of OAs as transients above a given limiting flux is derived and described with analytical expressions: in general OAs should appear as daily transients in optical surveys and as monthly/yearly transients in the mm/radio band. We find that ~2 OAyr-1 could already be detected by Gaia and up to 20 OAyr-1 could be observed by the ZTF survey. A larger number of 50 OAyr-1 should be detected by LSST in the optical band. For the X-ray band, ~26 OAyr-1 could be detected by the eROSITA. For the large population of OA detectable by LSST, the X-ray and optical follow up of the light curve (for the brightest cases) and/or the extensive follow up of their emission in the mm and radio band could be the key to disentangling their GRB nature from other extragalactic transients of comparable flux density. [ABSTRACT FROM AUTHOR]

We compare the prompt intrinsic spectral properties of a sample of short gamma-ray bursts (GRBs) with the first 0.3 s (rest frame) of long GRBs observed by Fermi/GBM (Gamma Burst Monitor). We find that short GRBs and the first part of long GRBs lie on the same Ep–Eiso correlation, that is parallel to the relation for the time-averaged spectra of long GRBs. Moreover, they are indistinguishable in the Ep–Liso plane. This suggests that the emission mechanism is the same for short and for the beginning of long events, and both short and long GRBs are very similar phenomena, occurring on different time-scales. If the central engine of a long GRB would stop after ∼0.3 × (1 + z) s, the resulting event would be spectrally indistinguishable from a short GRB. [ABSTRACT FROM AUTHOR]

We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to 2013 June. Inspired by the criteria we used to build a similar sample of bright long GRBs (the BAT6 sample), we selected SGRBs with favourable observing conditions for the redshift determination on ground, ending up with a sample of 36 events, almost half of which with a redshift measure. The redshift completeness increases up to about 70 per cent (with an average redshift value of z = 0.85) by restricting to those events that are bright in the 15–150 keV Swift Burst Alert Telescope energy band. Such flux-limited sample minimizes any redshift-related selection effects, and can provide a robust base for the study of the energetics, redshift distribution and environment of the Swift bright population of SGRBs. For all the events of the sample, we derived the prompt and afterglow emission in both the observer and (when possible) rest frame and tested the consistency with the correlations valid for long GRBs. The redshift and intrinsic X-ray absorbing column density distributions we obtain are consistent with the scenario of SGRBs originated by the coalescence of compact objects in primordial binaries, with a possible minor contribution (∼10–25 per cent) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity. [ABSTRACT FROM AUTHOR]

We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of gamma-ray burst afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power-law shape with index 2.4. However, for real instruments, this value depends on their low-energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 105-106 counts in the 0.3–10 keV band are needed to constrain the redshift with 10 per cent accuracy. As a test case, we discuss the XMM–Newton observation of GRB 090618 at z = 0.54. We are able to recover the correct redshift of this burst with the expected accuracy. [ABSTRACT FROM AUTHOR]

Aims. Long-duration gamma-ray bursts (GRBs) have been found to be associated with broad-lined type-Ic supernovae (SNe), but only a handful of cases have been studied in detail. Prompted by the discovery of the exceptionally bright, nearby GRB130427A (redshift z = 0.3399), we aim at characterising the properties of its associated SN2013cq. This is the first opportunity to test the progenitors of high-luminosity GRBs directly. Methods. We monitored the field of the Swift long-duration GRB130427A using the 3.6m TNG and the 8.2m VLT during the time interval between 3.6 and 51.6 days after the burst. Photometric and spectroscopic observations revealed the presence of the type Ic SN2013cq. Results. Spectroscopic analysis suggests that SN2013cq resembles two previous GRB-SNe, SN1998bw and SN2010bh, associated with GRB980425 and X-ray flash (XRF) 100316D, respectively. The bolometric light curve of SN2013cq, which is significantly affected by the host galaxy contribution, is systematically more luminous than that of SN2010bh (~2 mag at peak), but is consistent with SN1998bw. The comparison with the light curve model of another GRB-connected SN2003dh indicates that SN2013cq is consistent with the model when brightened by 20%. This suggests a synthesised radioactive 56Ni mass of ~0.4M⊙. GRB130427A/SN 2013cq is the first case of low-z GRB-SN connection where the GRB energetics are extreme (Eγ,iso ~ 1054 erg). We show that the maximum luminosities attained by SNe associated with GRBs span a very narrow range, but those associated with XRFs are significantly less luminous. On the other hand the isotropic energies of the accompanying GRBs span 6 orders of magnitude (1048 erg < Eγ,iso < 1054 erg), although this range is reduced when corrected for jet collimation. The GRB total radiated energy is in fact a small fraction of the SN energy budget. [ABSTRACT FROM AUTHOR]

Aims. We present the rest-frame light curves in the optical and X-ray bands of an unbiased and complete sample of the Swift long gamma-ray bursts (GRBs), namely, the BAT6 sample. Methods. The unbiased BAT6 sample (consisting of 58 events) has the highest level of completeness in redshift (∼95%), allowing us to compute the rest-frame X-ray and optical light curves for 55 and 47 objects, respectively. We compute the X-ray and optical luminosities, which accounte for any possible source of absorption (Galactic and intrinsic) that could affect the observed fluxes in these two bands. Results. We compare the behaviour observed in the X-ray to that in the optical bands to assess the relative contribution of the emission during the prompt and afterglow phases. We unarguably demonstrate that rest-frame optical luminosity distribution of the GRBs is not bimodal and is clustered around the mean value Log(LR) = 29.9 ± 0.8 when estimated at a rest-frame time of 12 h. This is in contrast to what is found in previous works and confirms that the GRB population has an intrinsic unimodal luminosity distribution. For more than 70% of the events, the rest-frame light curves in the X-ray and optical bands have a different evolution, indicating distinct emitting regions and/or mechanisms. The X-ray light curves, which are normalised to the GRB isotropic energy (Eiso), provide evidence for X-ray emission that is still powered by the prompt emission until late times (∼hours after the burst event). On the other hand, the same test performed for the Eiso-normalised optical light curves shows that the optical emission is a better proxy of the afterglow emission from early to late times. [ABSTRACT FROM AUTHOR]

We present a comprehensive statistical analysis of Swift X-ray light curves of gamma-ray bursts (GRBs) collecting data from more than 650 GRBs discovered by Swift and other facilities. The unprecedented sample size allows us to constrain the rest-frameX-ray properties of GRBs from a statistical perspective, with particular reference to intrinsic time-scales and the energetics of the different light-curve phases in a common rest-frame 0.3–30 keV energy band. Temporal variability episodes are also studied and their properties constrained. Two fundamental questions drive this effort: (i) Does the X-ray emission retain any kind of ‘memory’ of the prompt γ-ray phase? (ii) Where is the dividing line between long and short GRB X-ray properties? We show that short GRBs decay faster, are less luminous and less energetic than long GRBs in the X-rays, but are interestingly characterized by similar intrinsic absorption. We furthermore reveal the existence of a number of statistically significant relations that link the X-ray to prompt γ-ray parameters in long GRBs; short GRBs are outliers of the majority of these two-parameter relations. However and more importantly, we report on the existence of a universal three-parameter scaling that links the X-ray and the γ-ray energy to the prompt spectral peak energy of both long and short GRBs: EX, iso∝E1.00 ± 0.06γ, iso/E0.60 ± 0.10pk. [ABSTRACT FROM PUBLISHER]

ABSTRACT The comprehensive statistical analysis of Swift X-ray light curves, collecting data from six years of operation, revealed the existence of a universal scaling among the isotropic energy emitted in the rest-frame 1 - 104 keV energy band during the prompt emission ( [ABSTRACT FROM AUTHOR]

ABSTRACT The comprehensive statistical analysis of Swift X-ray light curves, collecting data from six years of operation, revealed the existence of a universal scaling among the isotropic energy emitted in the rest-frame 1 - 104 keV energy band during the prompt emission ( [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate whether there is any correlation between the X-ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma-ray bursts (GRBs) nearly complete in redshift (∼90 per cent). Being free of selection effects (except flux limit), this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray luminosities are computed at four different rest frame times (5 min, 1 h, 11 h and 24 h after trigger) and compared with the prompt emission isotropic energy Eiso, the isotropic peak luminosity Liso and the rest frame peak energy Epeak. We find that the rest frame afterglow X-ray luminosity do correlate with these prompt emission quantities, but the significance of each correlation decreases over time. This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time, with the prompt and afterglow emission dominating at early and late time, respectively. In particular, we found evidence that the plateau and the shallow decay phase often observed in GRB X-ray light curves are powered by activity from the central engine. The existence of the LX - Eiso correlation at late times ( [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, we present a sample of ten short gamma-ray bursts (GRBs) with a robust redshift determination, discovered by the Swift satellite up to 2011 January. We measure their X-ray absorbing column densities and collect data on the host galaxy offsets. We find evidence for intrinsic absorption and no correlation between the intrinsic absorbing column density and the projected offset of the GRB from its host galaxy centre. We find that the properties in the gamma regime ( T90, fluence and 1-s peak photon flux) of short GRBs with 'bright' and 'faint' X-ray afterglow likely disfavour different prompt emission mechanisms. The host galaxy offset and GRB duration ( T90) do not correlate. Instead, there is a hint of anticorrelation between the effective radius normalized host galaxy offset and T90. Finally, we examine the properties of short GRBs with short-lived and long-lived X-ray afterglows, finding that some short GRBs with short-lived X-ray afterglows have their optical afterglow detected. In light of this, the X-ray afterglow duration does not seem to be a unique indicator of a specific progenitor and/or environment for short GRBs. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the first systematic study of X-ray flare candidates in short gamma-ray bursts (SGRBs) exploiting the large 6-year Swift data base with the aim to constrain the physical nature of such fluctuations. We find that flare candidates appear in different types of SGRB host galaxy environments and show no clear correlation with the X-ray afterglow lifetime; flare candidates are detected both in SGRBs with a bright extended emission in the soft γ-rays and in SGRBs which do not show such component. We furthermore show that SGRB X-ray flare candidates only partially share the set of observational properties of long GRB (LGRB) flares. In particular, the main parameter driving the duration evolution of X-ray variability episodes in both classes is found to be the elapsed time from the explosion, with very limited dependence on the different progenitors, environments, central engine lifetimes, prompt variability time-scales and energy budgets. On the contrary, SGRB flare candidates significantly differ from LGRB flares in terms of peak luminosity, isotropic energy, flare-to-prompt luminosity ratio and relative variability flux. However, these differences disappear when the central engine time-scales and energy budget are accounted for, suggesting that (i) flare candidates and prompt pulses in SGRBs likely have a common origin; (ii) similar dissipation and/or emission mechanisms are responsible for the prompt and flare emission in LGRBs and SGRBs, with SGRBs being less energetic albeit faster evolving versions of the long class. Finally, we show that in strict analogy to the SGRB prompt emission, flares candidates fall off the lag-luminosity relation defined by LGRBs, thus strengthening the SGRB flare-prompt pulse connection. [ABSTRACT FROM AUTHOR]

Observations of Gamma-ray Bursts (GRBs) put forward in the recent years have revealed, with increasing evidence, that the historical classification between long and short bursts has to be revised. Within the Fireshell scenario, both short and long bursts are canonical bursts, consisting of two different phases. First, a Proper-GRB (P-GRB), that is the emission of photons at the transparency of the fireshell. Then, the Extended Afterglow, multiwavelength emission due to the interacion of the baryonic remnants of the fireshell with the CircumBurst Medium (CBM). We discriminate between long and short bursts by the amount of energy stored in the first phase with respect to the second one. Within the Fireshell scenario, we have introduced a third intermediate class: the disguised GRBs. They appear like short bursts, because their morphology is characterized by a first, short, hard episode and a following deflated tail, but this last part - coincident with the peak of the afterglow - is energetically predominant. The origin of this peculiar kind of sources is inferred to a very low average density of the environment (of the order of 10-3). After GRB 970228 and GRB 060614, we find in GRB 071227 a third example of disguised burst. [ABSTRACT FROM AUTHOR]

The analysis of various Gamma-Ray Bursts (GRBs) having a low energetics (an isotropic energy Eiso ≲ 1053 ergs) within the fireshell model has shown how the N(E) spectrum of their prompt emission can be reproduced in a satisfactory way by a convolution of thermal spectra. Nevertheless, from the study of very energetic bursts (Eiso ≳ 1054 ergs) such as, for example, GRB 080319B, some discrepancies between the numerical simulations and the observational data have been observed. We investigate a different spectrum of photons in the comoving frame of the fireshell in order to better reproduce the spectral properties of GRB prompt emission within the fireshell model. We introduce a phenomenologically modified thermal spectrum: a thermal spectrum characterized by a different asymptotic power-law index in the low energy region. Such an index depends on a free parameter α, so that the pure thermal spectrum corresponds to the case α = 0. We test this spectrum by comparing the numerical simulations with the observed prompt emission spectra of various GRBs. From this analysis it has emerged that the observational data can be correctly reproduced by assuming a modified thermal spectrum with α = -1.8. [ABSTRACT FROM AUTHOR]