Search

Whether binary neutron star mergers are the only astrophysical site of rapid neutron-capture process ($r$-process) nucleosynthesis remains unknown. Collapsars associated with long gamma-ray bursts (GRBs) and hypernovae are promising candidates. Simulations have shown that outflows from collapsar accretion disks can produce enough $r$-process materials to explain the abundances in the universe. However, there is no observational evidence to confirm this result at present. SN 2020bvc is a broad-lined type Ic (Ic-BL) supernova (SN) possibly associated with a low-luminosity GRB. Based on semi-analytic SN emission models with and without $r$-process materials, we perform a fitting to the multi-band light curves and photospheric velocities of SN 2020bvc. We find that in a $r$-process-enriched model the mixing of $r$-process materials slows down the photospheric recession and therefore matches the velocity evolution better. The fitting results show that $r$-process materials with mass of $\approx0.36~M_\odot$ and opacity of $\approx4~\rm cm^2~g^{-1}$ is needed to mix with about half of the SN ejecta. Our fitting results are weakly dependent on the nebular emission. Future statistical analysis of a sample of type Ic-BL SNe helps us understand the contribution of collapsars to the $r$-process abundance., 16 Pages, 9 Figures, 2 Tables, Accepted by ApJL

The environment of gamma-ray burst (GRB) has an important influence on the evolution of jet dynamics and of its afterglow. Here we investigate the afterglow polarizations in a stratified medium with the equal arrival time surface (EATS) effect. Polarizations of multi-band afterglows are predicted. The effects of the parameters of the stratified medium on the afterglow polarizations are also investigated. We found the influences of the EATS effect on the afterglow polarizations become important for off-axis detections and PD bumps move to later times with the EATS effect. Even the magnetic field configurations, jet structure and observational angles are fixed, polarization properties of the jet emission could still evolve. Here, we assume a large-scale ordered magnetic field in the reverse-shock region and a two-dimensional random field in the forward-shock region. Then PD evolution is mainly determined by the evolution of $f_{32}$ parameter (the flux ratio between the reverse-shock region and forward-shock region) at early stage and by the evolution of the bulk Lorentz factor $\gamma$ at late stage. Through the influences on the $f_{32}$ or $\gamma$, the observational energy band, observational angles, and the parameters of the stratified medium will finally affect the afterglow polarizations., Comment: 16 pages, 8 figures, accepted by ApJ

White-light superflares from ultra cool stars are thought to be resulted from magnetic reconnection, but the magnetic dynamics in a fully convective star is not clear yet. In this paper, we report a stellar superflare detected with the Ground Wide Angle Camera (GWAC), along with rapid follow-ups with the F60A, Xinglong 2.16m and LCOGT telescopes. The effective temperature of the counterpart is estimated to be $2200\pm50$K by the BT-Settl model, corresponding to a spectral type of L0. The $R-$band light curve can be modeled as a sum of three exponential decay components, where the impulsive component contributes a fraction of 23\% of the total energy, while the gradual and the shallower decay phases emit 42\% and 35\% of the total energy, respectively. The strong and variable Balmer narrow emission lines indicate the large amplitude flare is resulted from magnetic activity. The bolometric energy released is about $6.4\times10^{33}$ ergs, equivalent to an energy release in a duration of 143.7 hours at its quiescent level. The amplitude of $\Delta R=-8.6 $mag ( or $\Delta V=-11.2$ mag), placing it one of the highest amplitudes of any ultra cool star recorded with excellent temporal resolution. We argue that a stellar flare with such rapidly decaying and huge amplitude at distances greater than 1 kpc may be false positive in searching for counterparts of catastrophic events such as gravitational wave events or gamma-ray bursts, which are valuable in time-domain astronomy and should be given more attention., Comment: 9 pages, 5 figures, 1 table, MNRAS accepted

GRB 221009A is the brightest gamma-ray burst ever detected since the discovery of this kind of energetic explosions. However, an accurate measurement of the prompt emission properties of this burst is very challenging due to its exceptional brightness. With joint observations of \textit{Insight}-HXMT and GECAM-C, we made an unprecedentedly accurate measurement of the emission during the first $\sim$1800 s of GRB 221009A, including its precursor, main emission (ME, which dominates the burst in flux), flaring emission and early afterglow, in the hard X-ray to soft gamma-ray band from $\sim$ 10 keV to $\sim$ 6 MeV. Based on the GECAM-C unsaturated data of the ME, we measure a record-breaking isotropic equivalent energy ($E_{\rm iso}$) of $\bf \sim 1.5 \times 10^{55}$ erg, which is about eight times the total rest-mass energy of the Sun. The early afterglow data require a significant jet break between 650 s and 1100 s, most likely at $\sim950$ s from the afterglow starting time $T_{AG}$, which corresponds to a jet opening angle of $\sim {0.7^\circ} \ (\eta_\gamma n)^{1/8}$, where $n$ is the ambient medium density in units of $\rm cm^{-3}$ and $\eta_\gamma$ is the ratio between $\gamma$-ray energy and afterglow kinetic energy. The beaming-corrected total $\gamma$-ray energy $E_{\gamma}$ is $\sim 1.15 \times10^{51} \ (\eta_\gamma n)^{1/4}$ erg, which is typical for long GRBs. These results suggest that this GRB may have a special central engine, which could launch and collimate a very narrowly beamed jet with an ordinary energy budget, leading to exceptionally luminous gamma-ray radiation per unit solid angle. Alternatively, more GRBs might have such a narrow and bright beam, which are missed by an unfavorable viewing angle or have been detected without distance measurement., Comment: Submitted to National Science Review. This paper is under press embargo, contact the corresponding author for details

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio bursts,[1,2] of which the physical origin is still not fully understood. FRB 20201124A is one of the most actively repeating FRBs. In this paper, we present the collection of 1863 burst dynamic spectra of FRB 20201124A measured with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The current collection, taken from the observation during the FRB active phase from April to June 2021, is the largest burst sample detected for any FRB so far. The standard PSRFITs format is adopted, including dynamic spectra of the burst, and the time information of the dynamic spectra, in addition, mask files help readers to identify the pulse positions are also provided. The dataset is available in Science Data Bank, with the link https://www.doi.org/10.57760/sciencedb.j00113.00076.

Recently, the B.O.A.T. ("brightest of all time") gamma-ray burst, dubbed GRB 221009A, was detected by various instruments. Unprecedentedly, the GRB presented very-high-energy (VHE, energy above 0.1 TeV) gamma-ray emission with energy extending above 10 TeV, as reported by the Large High Altitude Air Shower Observatory (LHAASO). We here demonstrate that the VHE and especially >10 TeV emission may originate from the internal hadronic dissipation of the GRB, without the need of invoking any exotic processes as suggested by some previous studies. We also discuss the constraints on the properties of the GRB ejecta from multiwavelength and multi-messenger observations, which favors a magnetically dominated GRB ejecta. The suggested Poynting-flux-dominated GRB ejecta in this work supports the Blandford & Znajek (BZ) mechanism as the possible central engine model of GRB., 13 pages, 7 figures, 3 tables, accepted version

Despite decades of polarization observations and high-significance polarized $\gamma$-ray, X-ray, optical, and radio emissions in gamma-ray bursts (GRBs) have been accumulating in dozens of cases, people have yet to find a consistent scenario for understanding the globally observed timing properties of GRB polarization to date. Here, we report that the observed properties of GRB polarization exhibit a four-segment timing evolution at the cosmological distance: (I) an initial hump early on (within the first few seconds); (II) a later on power-law decay (from $\sim$10$^{1}$ to $\sim$10$^{4}$ s), which takes the form of $\pi_{\rm obs} \propto t^{-0.50 \pm 0.02}$; (III) afterwards a late-time rebrightening hump (from $\sim$10$^{4}$ to $\sim$10$^{5}$ s); and (IV) finally a flatting power-law decay (from $\sim$ 10$^{5}$ to $\sim$ 10$^{7}$ s), with the the form of $\pi_{\rm obs} \propto t^{-0.21 \pm 0.08}$. These findings may present a challenge to the mainstream of polarization models that assume the polarization time evolution change in different emission regions. We show that these results can be explained by relativistic and geometric effects of a highly relativistic and magnetized jet generated by the central engine, and "magnetic patches" distributed as a globally random but locally coherent form. Our analysis suggests that there is a single dominant mechanism that might account for the global observational properties of GRB polarization, and other emission mechanisms and effects might play a role in spatially local and temporally short effects on GRB polarization., Comment: 46 pages, 13 figures, 3 tables

Disponible preprint en: arXiv:1812.04023v1 [astro-ph.HE] [v1] Mon, 10 Dec 2018 19:00:52 UTC (4,376 KB), In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.© 2018, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature., This work was supported by the Royal Society, ERC Starting (Grant No. 639217), the European Union Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Global Fellowship (Grant No. 703916), the National Natural Science Foundation of China (Grant Nos. 11233001, 11773014, 11633007, 11403074, 11333005, 11503008, and 11590781), the National Basic Research Program of China (Grant No. 2015CB857100), NASA (Grant No. NNX13AD28A), an ARC Future Fellowship (Grant No. FT120100363), the National Science Foundation (Grant No. PHY-1430152), the Spanish MINECO (Grant No. AYA2016-76012-C3-1-P), the ICCUB (Unidad de Excelencia 'Maria de Maeztu') (Grant No. MDM-2014-0369), EU's Horizon Programme through a Marie Sklodowska-Curie Fellowship (Grant No. 702638), the Polish National Science Center (Grant Nos. 2015/17/B/ST9/03422, 2015/18/M/ST9/00541, 2013/10/M/ST9/00729, and 2015/18/A/ST9/00746), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA15020100), and the NWO Veni Fellowship (Grant No. 639.041.647).

We study 31 luminous, rapidly evolving optical transients (REOTs), and use the magnetar model and the ejecta--circumstellar-matter (CSM) interaction (CSI) model to fit their multiband light curves. We find that 28 events can be fitted by the magnetar model. and the derived masses ($\sim0.05$--5.58\,M$_\odot$) are consistent with that of stripped and ultrastripped core-collapse supernovae (CCSNe). On the other hand, the CSI model can fit 25 events; the derived ejecta masses are $\sim 3$--30\,M$_\odot$, consistent with that of SNe~Ibn. The results, together with the fact that many luminous REOTs have been confirmed to be luminous SNe~IIb/Ib/c or Ibn/IIn, suggest that at least a fraction of luminous REOTs spectroscopically unclassified might be rapidly evolving SNe~Ib/Ic or Ibn. Three events in the sample cannot be explained by the two models we use. We expect that future intense photometry, spectroscopic classifications, and systematic light-curve modeling of luminous REOTs will shed more light on their nature., Comment: 31 pages, 7 figures, 8 tables, the revised version of the paper submitted to ApJ

Since the LP-Nova models were proposed, and the short gamma-ray burst (SGRB) afterglows were confirmed, people have actively made searches for the evidence of the existence of LP-Novae among the optical (or near-infrared) counterparts of SGRBs. In this paper, we first summarize these observational progresses before 2012 in Section 2. In Section 3 and 4, we respectively introduce the basic properties of GRBs 130603B and 060614, as well as the theoretical interpretation for their near-infrared (NIR) counterparts, and their NIR excess may be the signature of the existence of LP-Novae. In Section 5, we describe the basic properties of GRB 080503, and the theoretical interpretation for its optical and X-ray counterparts, and the later re-brightening of its optical and X-ray light curves is explained as the ejecta radiation (merger-nova radiation) of magnetar heating after the neutron star merging. If the interpretations for the SGRB-associated optical and infrared counterparts are correct, they may provide the first series of direct evidence to show that SGRBs and some special LGRBs are originated from the compact star mergers. Besides LP-novae (and merger-novae), the high-speed orbital motion before the compact star merging and the merger itself will produce strong gravitational-wave bursts (GWBs). In the coming era of gravitational wave detection, the theoretical and observational studies on the electromagnetic counterparts of compact star mergers will receive more and more attentions. Due to the larger uncertainty of GWB's location, the LP-Novae associated with GWBs can serve as the best candidates for the precise location of GWBs. The fast developing high-cadence and wide-field optical-NIR surveys will make effective explorations on the LP-Novae and similar phenomena, and interact the detection and research of gravitational waves. Therefore, in the last section we present the methods for the future detections of LP-Novae, and the prospect of their multi-messenger detections.

The neutron-rich matter ejected by compact object mergers (neutron star-neutron star merger and neutron star-black hole merger) provides one of the most important environments for the syntheses of r-process elements. In recent seventeen years, theoretical studies suggested that the energy produced during the decay of r-process elements will form optical/near infrared (NIR) radiations after thermalization. This type of optical/NIR transients are called Li-Paczynski novae, or LP-novae for short. Since the typical peak brightness of LP-novae is ∼1000 times brighter than that of a typical nova, they are also called Kilonovae. Besides, both theoretical and observational studies have showed, directly or indirectly, that under certain conditions, the compact object mergers can produce the gamma-ray bursts with a rather short duration of T90 ≤ 2s (SGRBs for short), and most SGRBs may come from compact object mergers. After the identification of SGRB afterglows, dedicated searches of the LP-novae associated with SGRBs have been taken. In this review we present the theoretical progress of LP-novae in recent seventeen years, and the observational aspect will appear in a upcoming paper.

Neutron star-neutron star, neutron star-black hole, and black hole-black hole mergers are the most promising sources of high-frequency gravitational waves (GWs) in the universe. Simultaneously, these compact binary mergers can also produce various electromagnetic emission represented by short gamma-ray burst (SGRB) phenomena, which had been confirmed by the discoveries in the GW170817/GRB 170817A event. Therefore, these merger events are undoubtedly one of the most important targets of the multi-messenger astronomical researches. We provide a brief review on the state of art of the SGRB studies. First of all, the emission properties during the prompt, precursor, extended, and afterglow phases of SGRBs are described subsequently by following a theoretical understanding for relativistic jets. For completeness, we also pay many attentions to the associated kilonova/mergernova emission from a quasi-isotropic merger ejecta, the potential neutrino production, and the host galaxies of SGRBs, which can provide independent constraints on the origin of SGRBs. The observations in the GW170817/GRB 170817A event are specially introduced, in particular, focusing on the implications for the merger product and the jet structure. Finally, by considering of the viewing angle-dependence of SGRB emission, we discuss a new understanding of the luminosity function of SGRBs. Then, we can prospect the future observation by the GECAM satellites, which are specially designed to monitor the GW-associated SGRBs and planned to be launched in 2020.

Physical review / D 97(10), 103016 (2018). doi:10.1103/PhysRevD.97.103016, In a strong magnetic field, a high-energy photon can be absorbed and then produce an electron-positron pair. The produced electron/positron will in turn radiate a high-energy photon via synchrotron radiation, which then initiates a cascade. We built a one-dimensional Monte Carlo code to study the development of the cascade especially after it reaches the saturated status, when almost all the energy of the primary particles transfers to the photons. The photon spectrum in this status has a cutoff due to the absorption by magnetic fields, which is much sharper than the exponential one. Below the cutoff, the spectral energy distribution (SED) manifest itself as a broken power-law with a spectral index of 0.5 and 0.125, respectively, below and above the broken energy. The SED can be fitted by a simple analytical function, which is solely determined by the product of the cascade scale R and the magnetic field perpendicular to the motion of the particle B⊥, with an accuracy better than 96%. The similarity of the spectrum to that from the cascade in an isotropic black-body photon field is also studied., Published by APS, Woodbury, NY

A large-scale ordered magnetic field plays a very important role in the formation and acceleration of a gamma-ray burst (GRB) jet. During the GRB prompt phase, some dissipation processes may happen and disturb the magnetic field, making the field become random to some extent. Therefore, a mixed magnetic field consisting of an ordered component and a random component is plausible for the early afterglow phase. Here we investigate the polarization evolution and light curve of an afterglow under such a 3-dimensional mixed magnetic field. Three kinds of ordered component (i.e., aligned, toroidal and radial) are discussed. We find that the three cases are distinguishable through jet polarization evolution. The polarization angle for a 3D mixed magnetic field with an aligned ordered component can evolve gradually but only changes abruptly by $90^\circ$ in the toroidal and radial cases. Furthermore, during the reverse shock crossing time, the polarization degree (PD) can be non-zero for the toroidal case but roughly zero for the radial case. Since an aligned component in a jet corresponds to a magnetar central engine and a toroidal component corresponds to a black hole, GRB central engines are distinguishable through polarization observations even if the magnetic field is mixed in a jet. In addition, our polarization calculation can be applied to GRB 170817A associated with GW170817. Using the recently-observed PD upper limit $12\%$ of GRB 170817A at $t=244$ days, the magnetic field strength ratio of the ordered to random components in this afterglow is constrained to be $\lesssim0.9$., Comment: 26 pages, 7 figures, accepted by ApJ

The detection of the high-energy ($\sim290$ TeV) neutrino coincident with the flaring blazar TXS 0506+056, the first and only $3��$ neutrino-source association to date, provides new, multimessenger tests of the weak equivalence principle (WEP) and Lorentz invariance. Assuming that the flight time difference between the TeV neutrino and gamma-ray photons from the blazar flare is mainly caused by the gravitational potential of the Laniakea supercluster of galaxies, we show that the deviation from the WEP for neutrinos and photons is conservatively constrained to have an accuracy of $10^{-6}-10^{-7}$, which is 3--4 orders of magnitude better than previous results placed by MeV neutrinos from supernova 1987A. In addition, we demonstrate that the association of the TeV neutrino with the blazar flare sets limits on the energy scales of quantum gravity for both linear and quadratic violations of Lorentz invariance (LIV) to $E_{\rm QG, 1}>3.2\times10^{15}-3.7\times10^{16}$ GeV and $E_{\rm QG, 2}>4.0\times10^{10}-1.4\times10^{11}$ GeV. These improve previous limits on both linear and quadratic LIV energy scales in neutrino propagation by 5--7 orders of magnitude., 5 pages, 1 table, no figures. Accepted by Journal of High Energy Astrophysics

Recently, a hydrogen-poor superluminous supernova (SLSN) iPTF15esb at redshift $z=0.224$ was reported, whose light curve (LC) and spectrum show several unusual characteristics. Its late-time spectrum shows a strong, broad H$\alpha $ emission line and the bolometric LC exhibits two peaks and a post-peak plateau. Here we propose an ejecta-circumstellar medium (CSM) interaction model involving multiple shells and winds to explain this double-peak SN. We find that the theoretical LC reproduced by this model can well match the observations of iPTF15esb. Based on this result, we infer that the progenitor has undergone at least three violent mass-loss processes before the SN explosion. Furthermore, we find that the masses of the CSM wind and shells from the outermost shell (the first eruption) to the innermost wind (the final eruption) decrease sequentially but their densities increase. The variation trend of the inferred densities of the shells and wind is consistent with the stellar structure before an SN explosion. Further investigations for similar SLSNe would provide a probe for the mass-loss history of their progenitors., Comment: 6 pages, 2 figures, 2 tables

A binary neutron star (BNS) merger has been widely argued to be one of the progenitors of a short gamma-ray burst (SGRB). This central engine can be verified if its gravitational-wave (GW) event is detected simultaneously. Once confirmed, this kind of association will be a landmark in multi-messenger astronomy and will greatly enhance our understanding of the BNS merger processes. Due to the limited detection horizon of BNS mergers for the advanced LIGO/Virgo GW observatories, we are inclined to local SGRBs within few hundreds of mega-parsecs. Since normal SGRBs rarely fall into such a close range, to make it more observationally valuable, we have to focus on low-luminosity SGRBs which have a higher statistical occurrence rate and detection probability. However, there is a possibility that an observed low-luminosity SGRB is intrinsically powerful but we are off-axis and only observe its side emission. In this paper, we provide some theoretical predictions of both the off-axis afterglow emission from a nearby SGRB under the assumption of a structured jet and the macronova signal from the ejecta of this GW-detectable BNS merger. From the properties of the afterglow emission, we could distinguish an off-axis normal SGRB from an intrinsically low-energy quasi-isotropic class. Furthermore, with follow-up multi-wavelength observations, a few parameters for BNS mergers (e.g. the medium density and the ejecta mass and velocity) would be constrained., Comment: 7 pages, 9 figures

We present a detailed report of the connection between long-duration gamma-ray bursts (GRBs) and their accompanying supernovae (SNe). The discussion presented here places emphasis on how observations, and the modelling of observations, have constrained what we know about GRB-SNe. We discuss their photometric and spectroscopic properties, their role as cosmological probes, including their measured luminosity–decline relationships, and how they can be used to measure the Hubble constant. We present a statistical summary of their bolometric properties and use this to determine the properties of the “average” GRB-SN. We discuss their geometry and consider the various physical processes that are thought to power the luminosity of GRB-SNe and whether differences exist between GRB-SNe and the SNe associated with ultra-long-duration GRBs. We discuss how observations of their environments further constrain the physical properties of their progenitor stars and give a brief overview of the current theoretical paradigms of their central engines. We then present an overview of the radioactively powered transients that have been photometrically associated with short-duration GRBs, and we conclude by discussing what additional research is needed to further our understanding of GRB-SNe, in particular the role of binary-formation channels and the connection of GRB-SNe with superluminous SNe.

We work on a GRB sample whose initial Lorentz factors ($\Gamma_0$) are constrained with the afterglow onset method and the jet opening angles ($\theta_{\rm j}$) are determined by the jet break time. We confirm the $\Gamma_0$ - $E_{\gamma,\rm iso}$ correlation by Liang et al. (2010), and the $\Gamma_0$ - $L_{\gamma,\rm iso}$ correlation by L{\"u} et al. (2012). Furthermore, we find correlations between $\Gamma_0$ and the beaming corrected $\gamma$-ray energy ($E_{\gamma}$) and mean $\gamma$-ray luminosity ($L_{\gamma}$). By also including the kinetic energy of the afterglow, we find rough correlations (with larger scatter) between $\Gamma_0$ and the total ($\gamma$-ray plus kinetic) energy and the total mean luminosity, both for isotropic values and beaming corrected values: these correlations allow us to test the data with GRB central engine models. Limiting our sample to the GRBs that likely have a black hole central engine, we compare the data with theoretical predictions of two types of jet launching mechanisms from BHs, i.e. the non-magnetized $\nu \bar\nu$ - annihilation mechanism, and the strongly magnetized Blandford-Znajek (BZ) mechanism. We find that the data are more consistent with the latter mechanism, and discuss the implications of our findings for GRB jet composition., Comment: 12 pages; 3 figures; 2 tables; accepted by Journal of High Energy Astrophysics

We propose to construct a 2.5 m wide-field and high-resolution telescope (WeHiT).It will be the first facility in the world with a special innovative design and can conduct bothhigh-resolution solar observations and the wide-field night sky survey. Its scientific objectivescover solar physics, time-domain astronomy, galactic astrophysics, and exoplanet science, all ofwhich are recent hot topics in astronomy. The telescope can provide top-ranked data for Chineseastronomers. In particular, as the largest on-axis solar telescope in the world with wide-field thanall large solar telescopes operating at present, it can provide unprecedented high-resolution solardata, which can help us obtain breakthrough achievements in the study of solar active regionsand solar eruptions, such as solar flares and coronal mass ejections (CMEs). The data are also veryuseful for the study of space weather. As a large telescope in the China time-zone, the telescopeis very unique in continuously monitoring short-time transient events by filling the gap in thetime-zone. Furthermore, it can provide valuable high-quality data of the narrow-band extragalacticsurvey, nearby galaxy study and exoplanet survey. Moreover, the telescope is a top-level one whichcan make essential contribution to training the graduate and undergraduate students majoring inobservational astronomy.

The “strange star - NDAF” model (NDAF: Neutrino Dominated Accretion Flow) is proposed as an alternative central engine of gamma-ray bursts for unifying the interpretation of the prompt emission and postburst activities of gamma-ray bursts. The structure of NDAF around a strange star is calculated. Different from other central compact objects, the strange star will feed back the phase transition energy of strangization on the accretion flow, with neutrinos as energy carriers. The friction between NDAF and strange star is ignored in this paper. The results indicate: firstly, the structure of NDAF around a strange star is sensitive to accretion rate; secondly, if accretion rate is larger than 0.18 Mʘ s-1, the “strange star - NDAF” model can unify the explanation on the prompt emission and postburst activities of gamma-ray bursts, and the range of allowable accretion rates is wider than that in frictionless “neutron star - NDAF” models; thirdly, the range of annihilation energy of “strange star - NDAF” model is very wide, when the accretion rate is higher than 0.3 Mʘ s-1, the annihilation energy is greater than 1051 erg; finally, if the accretion rate is greater than 0.3 Mʘ s-1, the annihilation energy of “strange star - NDAF” model is larger than what of “black hole - NDAF” model at the same accretion rate by more than one order of magnitude, it is favorable to explaining some extremely energetic gamma-ray bursts.

The gravitational wave (GW) signal from a binary neutron star (NS) merger wasdetected by the LIGO/virgo detectors on August 17th 2017 for the first time.A short gamma-ray burst (GRB) and a kilonova were detected 1.74 s andlinebreak 10.9 h after the GW signal, respectively.These detections confirm the hypothesis that at least some short gamma-ray bursts (GRB)originate from binary neutron star mergers.Furthermore, it is clear that r-process nucleosynthesis should occur in NS-NS mergers and then power a kilonova.Observations on its multi-wavelength counterparts (including the prompt gamma-ray emission, kilonova, and afterglow) make us be ableto investigate their physical origins as a whole for the first time.Comparisons between the theories and observations would greatly help answerquestions such as the binary NS merger remnant, the characteristics of the GRB jet,and the characteristics of the ejecta. This paper summarizes these electromagnetic counterparts to GW170817 from the NS-NS merger and provides theoretical interpretations for them.

Gamma-ray bursts (GRBs) are the most violent explosions in the universe. Thanks totheir extreme brightness, GRBs can be detected up to the edge of the visible universe.As bright beacons in the deep universe, high-redshift GRBs have been considered as an ideal tool toexplore the properties of the early universe: including the dark energy and cosmological parameters,star formation rate, first stars, the reionization and metal enrichment history of the universe.So the detection of high-$z$ GRBs has important scientific significance. Compared to currentmissions, Einstein Probe has a higher sensitivity and a wider field-of-view, operating the softX-ray band (0.5–4 keV), which would be suitable for the detection of high-$z$ GRBs. With itsobservational mode and ability, we compute the expected detection rate of high-$z$ GRBs byEinstein Probe by means of a observational tested population synthesis model of Swift GRBs.Our results show that the detection rates are expected to be about 20 events $\\rm~yr^{-1}$ $\\rm~sr^{-1}$for $z>6$ bursts, 6 events $\\rm~yr^{-1}$ $\\rm~sr^{-1}$ for $z>8$ bursts, and 1 events $\\rm~yr^{-1}$$\\rm~sr^{-1}$ for $z>12$ bursts, respectively. Over the 3 yr lifetime of the mission,Einstein Probe will able to detect about 65 GRBs at $z>6$, including $\\sim20$ GRBs at $z>8$ and$\\sim3$ GRBs at $z>12$. In sum, Einstein Probe would significantly improve the detection of high-$z$GRBs, and these abundant observational information would probably reveal some scientific mysteries ofthe early universe.

Conversion of neutron stars with different equations of state to strange stars with different equations of state is studied in this paper. For a hot neutron star, the conversion could lead to large changes in the period of the pulsar. The magnitude of the change is largely determined by the equation of state of neutron matter and the duration of the change is strongly dependent on the equation of state of strange matter. For a cold neutron star, the conversion could result in a γ-ray burst.

The increasingly deep limit on the neutrino emission from gamma-ray bursts (GRBs) with IceCube observations has reached the level that could put useful constraints on the fireball properties. We first present a revised analytic calculation of the neutrino flux, which predicts a flux an order of magnitude lower than that obtained by the IceCube collaboration. For benchmark model parameters (e.g. the bulk Lorentz factor is \Gamma=10^{2.5}, the observed variability time for long GRBs is t_v=0.01 s and the ratio between the energy in accelerated protons and in radiation is \eta_p=10 for every burst) in the standard internal shock scenario, the predicted neutrino flux from 215 bursts during the period of the 40-string and 59-string configurations is found to be a factor of ~3 below the IceCube sensitivity. However, if we accept the recently found inherent relation between the bulk Lorentz factor and burst energy, the expected neutrino flux increases significantly and the spectral peak shifts to lower energy. In this case, the non-detection then implies that the baryon loading ratio should be \eta_p4x10^{12} cm assuming the same dissipation radius for every burst and benchmark parameters for fireballs. We also calculate the diffuse neutrino flux from GRBs for different luminosity functions existing in the literature. The expected flux exceeds the current IceCube limit for some luminosity functions, and thus the non-detection constrains \eta_p, Comment: Accepted by ApJ, 14 pages, 5 figures, typos corrected, scheduled for the June 10, 2012, v752 - 1 issue

Low-luminosity gamma-ray bursts (GRBs) with luminosity . 10^49erg/s probably consititute a distinct population from the classic high-luminosity GRBs. They are the most luminous objects detected so far within ~ 100 Mpc, the horizon distance of ultra-high energy cosmic rays (UHECRs), so they are considered to be candidate sources of UHECRs. It was recently argued that the energy production rate in UHECRs is much larger than that in gamma-ray photons of long GRBs measured by the Fermi satellite, which, if true, would challenge the view that GRBs can be the sources of UHECRs. We here suggest that many of the low luminosity GRBs, due to their low luminosity, can not trigger the current GRB detectors and hence their contribution to the local gamma-ray energy production rate is missing. We find that the real local energy production rate by low-luminosity GRBs, taking into account the missing part, which constitutes a dominant fraction of the total amount, could be sufficient to account for the flux of UHECRs. Due to the low-luminosity, only intermediate-mass or heavy nuclei can be accelerated to ~ 10^20 eV. We discuss the acceleration and survival of these UHE nuclei in low-luminosity GRBs, especially in those missing low-luminosity GRBs. At last, the accompanying diffuse neutrino flux from the whole low-luminosity GRB population is calculated., 10 pages, 5 figures, accepted for publication in MNRAS

In an effort to understand the puzzle of classifying gamma-ray bursts (GRBs), we perform a systematic study of {\it Swift} GRBs and investigate several short GRB issues. Though short GRBs have a short ($\lesssim2$ s) prompt duration as monitored by the Burst Alert Telescope, the composite light curves including both the prompt and afterglow emission suggest that most of the short GRBs have a similar radiative feature to long GRBs. Further, some well-studied short GRBs might also have an intrinsically long prompt duration, which renders them as a type of short GRB imposters. Genuine short GRBs detected by {\it Swift} might be rare that discriminating the observed short GRBs is, not surprisingly, troublesome. In particular, the observational biases in the host identification and redshift measurement of GRBs should be taken with great caution. The redshift distribution which has been found to be different for long and short GRBs might have been strongly affected by the measurement methods. We find that the redshifts measured from the presumed host galaxies of long and short GRBs appear to have a similar distribution., Updated to match the version published in ApJ, 11 pages, 6 figues, 2 tables

In this paper we calculate and discuss the composition and equation of state of the outer shell of a neutron star composed of cold catalyzed matter. We consider the contributions by the lattice energy and exchange energy of a uniform electron gas under a strong magnetic field. Our conclusions are: (1) a strong magnetic field softens the equation of state for low density regions; (2) it has almost no effect in high density regions; (3) the nuclear mass formula has a more marked effect on the composition of the outer shell.

In this paper we present the spectrum of synchro-curvature self-Compton (SCSC) radiation of relativistic electrons with a power-law distribution of Lorentz factors. We find that the resulting spectrum is significantly different from that of either synchrotron self-Compton or curvature self-Compton radiation if both the curvature radius of the magnetic field and the cyclotron radius of the electrons are within some proper ranges. The effects of electrons' cooling and drifting, the low-energy self absorption in seed spectra, and the Klein-Nishina cutoff are also discussed, in order to get an accurate picture. We take gamma-ray bursts (GRBs) as our example environment for discussions. The results would be considered as a universal approach of the self-Compton emission of relativistic electrons moving in curved magnetic fields, and thus could be applied to many astrophysical phenomena, including GRBs, active galactic nuclei (AGNs), and pulsars., 9 pages, 11 figures, accepted for publication in MNRAS

Afterglows from jetted gamma-ray bursts are generally believed to be characterized by an obvious break in the light curve at the relativistic stage. We show that it is not the case. However, an obvious break does exist at the transition from the relativistic phase to the non-relativistic phase. Although this break itself is parameter-dependent, afterglows from jetted remnant are uniformly characterized by a quick decay during the non-relativistic phase.

The observations of a nearby low-luminosity gamma-ray burst (GRB) 060218 associated with supernova SN 2006aj may imply an interesting astronomical picture where a supernova shock breakout locates behind a relativistic GRB jet. Based on this picture, we study neutrino emission for early afterglows of GRB 060218-like GRBs, where neutrinos are expected to be produced from photopion interactions in a GRB blast wave that propagates into a dense wind. Relativistic protons for the interactions are accelerated by an external shock, while target photons are basically provided by the incoming thermal emission from the shock breakout and its inverse-Compton scattered component. Because of a high estimated event rate of low-luminosity GRBs, we would have more opportunities to detect afterglow neutrinos from a single nearby GRB event of this type by IceCube. Such a possible detection could provide evidence for the picture described above., 6 pages, 2 figures, accepted for publication in MNRAS

Since the equilibrium composition of strange matter is crucial to the accurate calculation of its neutrino energy loss rates, we determine the composition by using the thermodynamic potentials (per unit volume) of the strange matter, and find that the electron abundance decreases with the strong interaction coupling constant or ba-ryon density. After deriving the formula of neutrino energy loss rates of the strange matter, we study the cooling of a homogeneous strange star with a thin crust in detail, and compare it with that of a neutron star. We conclude that the surface temperature of a young strange star is much less than that of a young neutron star of the same age, so one can distinguish between a strange star and a neutron star.

The phase transition of two-flavor quark matter into strange matter in the interior of a neutron star is studied. It is shown that the timescale of the transition is less than 0.1μs and the s-quark semileptonic processes are of significance in neutrino emission during the transition. Owing to these processes, the transition in a whole neutron star will liberate several 1051 erg of energy as a neutrino burst.

We report on the results of rapid responses to GRB 080330 and GRB 080413A by the ROTSE-IIIb and ROTSE-IIIc telecopes. In both cases, optical follow-up started during the prompt emission phase and recorded optical flares peaking at similar times as the last hard X-ray pulses detected by the Swift BAT. For both events, the XRT observations started at the end of the prompt BAT detection and displayed an evolution in time opposite to the optical lightcurve. These very early observations provide us an opportunity to probe the onset of afterglow for these two events.

We present here the photometric results of the Robotic Palomar 60-inch Telescope (P60) observations for GRB optical afterglows. The quick response time and the size of the telescope allows us to observe the optical transients through V, R, I, g', i', z' filters, sometimes as early as 0.01 day after the trigger. Comparing the data obtained with the various filters we were able to compute color indices for the 1 day after the trigger epoch. The observations lasts until the OT fades below the detection threshold.