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Giant flares are the brightest and rarest outbursts from magnetars, with isotropic energies of 1044–1046 erg. GRB 200415A is suggested as a magnetar giant flare from NGC 253. Fermi Large Area Telescope detected the GeV afterglow emission from the flare, which is the first time that the GeV emission is detected from a giant flare. In this paper, we study the multiwavelength afterglow radiation of electrons accelerated by the forward and reverse shocks, produced via interactions between the ejecta and the circumburst medium in the afterglow phase of GRB 200415A. We found that in the GeV band, the forward shock emission is usually higher than the reverse shock emission, and can explain observations well, while in the optical and infrared bands, contributions from the forward and reverse shocks can be comparable for reasonable parameter sets, and the brightness of the forward-shock emission can reach 20 mag (AB). We predict that future telescopes such as Wide Field Survey Telescope, Chinese Space Station Telescope, the Large Synoptic Survey Telescope, and James Webb Space Telescope can detect the optical and infrared afterglow emission from giant flares similar to GRB 200415A. In addition, we consider two cases where the ejecta are dominated by protons or electron–positron pairs. We find that the reverse shock emission is comparable in these two cases for a hard electron spectrum, while for a soft electron spectrum, the reverse shock emission is much weaker in the pair-dominating case..

We present the broadband numerical modeling of afterglows for two remarkably bright long gamma-ray bursts (GRBs), GRB 050820A and GRB 070125, with a wide range of observations from the radio band to the X-ray band. In our work, we fit light curves and constrain physical parameters using a standard forward shock model from the afterglowpy Python package, considering different jet structures and the jet lateral expansion. For GRB 050820A, the constrained jet is close to a top-hat jet with an extremely small half opening angle of about 0.015 rad, and the circumburst matter density is as small as 10−7 cm−3, which suggests that this peculiar long GRB might originate from metal-poor stars with low mass-loss rates. To explain the late time optical light curves of GRB 070125, the effects of the lateral expansion and the participation factor of electrons that are accelerated by the shock have to be taken into account. The constrained results for GRB 070125 show that the jet is also close to a top-hat jet with a half opening angle of about 0.1 rad, the viewing angle is about 0.05 rad, the circumburst density is about 10 cm−3, and the participation factor is about 0.1. The jet energy of the two bursts is required to be ∼1051–1052 erg, which can be produced by a millisecond magnetar or a hyper-accreting black hole.

The astrophysical journal 913(2), L33 (2021). doi:10.3847/2041-8213/abf925, The unidentified TeV source MGRO J1908+06, with emission extending from hundreds of GeV to beyond 100 TeV, is one of the most intriguing sources in the Galactic plane. MGRO J1908+06 spatially associates with an IceCube hotspot of neutrino emission. Although the hotspot is not significant yet, this suggests a possible hadronic origin of the observed gamma-ray radiation. Here we describe a multiwavelength analysis on MGRO J1908+06 to determine its nature. We identify, for the first time, an extended GeV source as the counterpart of MGRO J1908 + 06, discovering possibly associated molecular clouds (MCs). The GeV spectrum shows two well-differentiated components: a soft spectral component below ∼10 GeV, and a hard one (Γ ∼ 1.6) above these energies. The lower-energy part is likely associated with the dense MCs surrounding the supernova remnant (SNR) G40.5−0.5, whereas the higher-energy component, which connects smoothly with the spectrum observed in TeV range, resembles the inverse Compton emission observed in relic pulsar wind nebulae. This simple scenario seems to describe the data satisfactorily, but raises questions about the interpretation of the emission at hundreds of TeV. In this scenario, no detectable neutrino flux would be expected., Published by Univ., Chicago, Ill. [u.a.]

The jet breaks in the afterglow lightcurves of short gamma-ray bursts (SGRBs), rarely detected so far, are crucial for estimating the half-opening angles of the ejecta ($\theta_{\rm j}$) and hence the neutron star merger rate. In this work we report the detection of jet decline behaviors in GRB 150424A and GRB 160821B and find $\theta_{\rm j}\sim 0.1$ rad. Together with five events reported before 2015 and other three "identified" recently (GRB 050709, GRB 060614 and GRB 140903A), we have a sample consisting of nine SGRBs and one long-short GRB with reasonably estimated $\theta_{\rm j}$. In particular, three {\it Swift} bursts in the sample have redshifts $z\leq 0.2$, with which we estimate the local neutron star merger rate density {to be $\sim 1109^{+1432}_{-657}~{\rm Gpc^{-3}~yr^{-1}}$ or $162^{+140}_{-83} {\rm Gpc^{-3}yr^{-1}}$ if the narrowly-beamed GRB 061201 is excluded}. Inspired by the typical $\theta_{\rm j}\sim 0.1$ rad found currently, we further investigate whether the off-beam GRBs (in the uniform jet model) or the off-axis events (in the structured jet model) can significantly enhance the GRB/GW association or not. For the former the enhancement is at most moderate, while for the latter the enhancement can be much greater and a high GRB/GW association probability of $\sim 10\%$ is possible. We also show that the data of GRB 160821B may contain a macronova/kilonova emission component with a temperature of $\sim 3100$ K at $\sim 3.6$ days after the burst and more data are needed to ultimately clarify., Comment: 14 pages, 8 figures, accepted for publication in ApJ

We report the detection of a GeV $\gamma$-ray source which is spatially overlapping and thus very likely associated with the unidentified very-high-energy (VHE) $\gamma$-ray source HESS J1427-608 with the Pass 8 data recorded by the Fermi Large Area Telescope. The photon spectrum of this source is best described by a power-law with an index of $1.85\pm0.17$ in the energy range of $3-500$ GeV, and the measured flux connects smoothly with that of HESS J1427-608 at a few hundred GeV. This source shows no significant extension and time variation. The broadband GeV-TeV emission over four decades of energies can be well fitted by a single power-law function with an index of 2.0, without obvious indication of spectral cutoff toward high energies. Such a result implies that HESS J1427-608 may be a PeV particle accelerator. We discuss possible nature of HESS J1427-608 according to the multi-wavelength spectral fittings. Given the relatively large errors, either a leptonic or a hadronic model can explain the multi-wavelength data from radio to VHE $\gamma$-rays. The inferred magnetic field strength is a few $\mu$G, which is smaller than typical values of supernova remnants (SNRs), and is consistent with some pulsar wind nebulae (PWNe). On the other hand, the flat $\gamma$-ray spectrum is slightly different from typical PWNe but similar to that of some known SNRs., Comment: 8 pages, 8 figures, 2 tables, Accepted for publication in ApJ

GRB 160625B is an extremely-bright outburst with well-monitored afterglow emission. The geometry-corrected energy is high up to $\sim 5.2\times10^{52}$ erg or even $\sim 8\times 10^{52}$ erg, rendering it the most energetic GRB prompt emission recorded so far. We analyzed the time-resolved spectra of the prompt emission and found that in some intervals there were likely thermal-radiation components and the high energy emission were characterized by significant cutoff. The bulk Lorentz factors of the outflow material are estimated accordingly. We found out that the Lorentz factors derived in the thermal-radiation model are consistent with the luminosity-Lorentz factor correlation found in other bursts as well as in GRB 090902B for the time-resolved thermal-radiation components. While the spectral cutoff model yields much lower Lorentz factors that are in tension with the constraints set by the electron pair Compoton scattering process. We then suggest that these spectral cutoffs are more likely related to the particle acceleration process and that one should be careful in estimating the Lorentz factors if the spectrum cuts at a rather low energy (e.g., $\sim$ tens MeV). The nature of the central engine has also been discussed and a stellar-mass black hole is likely favored., Comment: Published in ApJ on 9 Feb 2017

GRB 131231A was detected by the Large Area Telescope onboard Fermi Space Gamma-ray Telescope. The high energy gamma-ray ($> 100$ MeV) afterglow emission spectrum is $F_��\propto ��^{-0.54\pm0.15}$ in the first $\sim 1300$ s after the trigger and the most energetic photon has an energy $\sim 62$ GeV arriving at $t\sim 520$ s. With reasonable parameters of the GRB outflow as well as the density of the circum-burst medium, the synchrotron radiation of electrons or protons accelerated at an external forward shock have difficulty accounting for the data. The synchrotron self-Compton radiation of the forward shock-accelerated electrons, instead, can account for both the spectrum and temporal behavior of the GeV afterglow emission. We also show that the prospect for detecting GRB 131231A$-$like GRBs with Cherenkov Telescope Array (CTA) is promising., ApJL in press

The existence of fast radio bursts (FRBs), a new type of extragalatic transients, has been established recently and quite a few models have been proposed. In this work we discuss the possible connection between the FRB sources and ultra-high energy ($>10^{18}$ eV) cosmic rays. We show that in the blitzar model and the model of merging binary neutron stars, the huge energy release of each FRB central engine together with the rather high rate of FRBs, the accelerated EeV cosmic rays may contribute significantly to the observed ones. In other FRB models including for example the merger of double white dwarfs and the energetic magnetar radio flares, no significant EeV cosmic ray is expected. We also suggest that the mergers of double neutron stars, even if they are irrelevant to FRBs, may play a non-ignorable role in producing EeV cosmic ray protons if supramassive neutron stars were formed in a good fraction of mergers and the merger rate is $\gtrsim 10^{3}~{\rm yr^{-1}~ Gpc^{-3}}$. Such a possibility will be unambiguously tested in the era of gravitational wave astronomy., Fig.1 is added to show the possibility distribution, as a function of maximal gravitational mass, of forming supramassive neutron stars in the double neutron star mergers. Major conclusions are unchanged

Long‐lived high‐energy (>100 MeV) emission, a common feature of most Fermi‐LAT detected gamma‐ray burst, is detected up to ∼102 s in the short GRB 090510. We study the origin of this long‐lived high‐energy emission, using broad‐band observations including X‐ray and optical data. We confirm that the late >100 MeV, X‐ray and optical emission can be naturally explained via synchrotron emission from an adiabatic forward shock propagating into a homogeneous ambient medium with low number density under significant Klein‐Nishina effects. Under the constraints from the low‐energy observations, the adiabatic forward shock synchrotron emission is consistent with the later‐time (t>2 s) high‐energy emission, but falls below the early‐time (t

Recently, Fermi-LAT detected GeV emission during the X-ray flaring activity in GRB 100728A. We study various scenarios for its origin. The hard spectrum of the GeV emission favors the external inverse-Compton origin in which X-ray flare photons are up-scattered by relativistic electrons in the external forward shock. This external IC scenario, with anisotropic scattering effect taken into account, can reproduce the temporal and spectral properties of the GeV emission in GRB 100728A., Comment: Minor revisions, 6 pages, 2 figures, accepted for publication in ApJ

Gamma‐ray bursts have been detected at photon energies up to tens of GeV. We review some recent developments in the X‐ray to GeV photon phenomenology in the light of Swift and Fermi observations, and some of the theoretical models developed to explain them.

We calculate the high energy afterglow emission from short Gamma-Ray Bursts (SGRBs) in the external shock model. There are two possible components contributing to the high energy afterglow: the electron synchrotron emission and the synchrotron self-Compton (SSC) emission. We find that for typical parameter values of SGRBs, the early high-energy afterglow emission in 10 MeV-10 GeV is dominated by the synchrotron emission. For a burst occurring at redshift z =0.1, the high-energy emission can be detectable by Fermi LAT if the blast wave has an energy E>=10^51 ergs and the fraction of energy in electrons is \epsilon_e>=0.1 . This provides a possible explanation for the high energy tail of SGRB 081024B., Comment: 5 pages, 5 figures. This is a slightly expanded version of the paper that will appear in Science in China Series G

GRB 130821A was detected by Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL, RHESSI and Mars Odyssey-HEND. Although the data of GRB 130821A are very limited, we show in this work that the high energy gamma-ray emission (i.e., above 100 MeV) alone imposes tight constraint on the density profile of the circum-burst medium as well as the initial Lorentz factor of the outflow. The temporal behavior of the high energy gamma-ray emission is consistent with the forward shock synchrotron radiation model and the circum-burst medium likely has a constant-density profile. The Lorentz factor is about a few hundred, similar to other bright GRBs., 7 pages, 4 figures, 2 tables, ApJ, in press

A nearby super-luminous burst GRB 130427A was simultaneously detected by six $\gamma$-ray space telescopes ({\it Swift}, Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL, AGILE and RHESSI) and by three RAPTOR full-sky persistent monitors. The isotropic $\gamma-$ray energy release is of $\sim 10^{54}$ erg, rendering it the most powerful explosion among the GRBs with a redshift $z\leq 0.5$. The emission above 100 MeV lasted about one day and four photons are at energies greater than 40 GeV. We show that the count rate of 100 MeV-100 GeV emission may be mainly accounted for by the forward shock synchrotron radiation and the inverse Compton radiation likely dominates at GeV-TeV energies. In particular, an inverse Compton radiation origin is established for the $\sim (95.3,~47.3,~41.4,~38.5,~32)$ GeV photons arriving at $t\sim (243,~256.3,~610.6,~3409.8,~34366.2)$ s after the trigger of Fermi-GBM. Interestingly, the external-inverse-Compton-scattering of the prompt emission (the second episode, i.e., $t\sim 120-260$ s) by the forward-shock-accelerated electrons is expected to produce a few $\gamma-$rays at energies above 10 GeV, while five were detected in the same time interval. A possible unified model for the prompt soft $\gamma-$ray, optical and GeV emission of GRB 130427A, GRB 080319B and GRB 090902B is outlined. Implication of the null detection of $>1$ TeV neutrinos from GRB 130427A by IceCube is discussed., Comment: 10 pages including 4 figures and 2 tables. Section 3.2 is added to discuss the limitation of alternative models for the >5 GeV afterglow emission. To appear in ApJ

Ultra-luminous infrared galaxies (ULIRGs) are the most luminous and intense starburst galaxies in the Universe. Both their star-formation rate (SFR) and gas surface mass density are very high, implying a high supernovae rate and an efficient energy conversion of energetic protons. A small fraction of these supernovae is the so-called hypernovae with a typical kinetic energy ~1e52 erg and a shock velocity >=1e9 cm/s. The strong shocks driven by hypernovae are able to accelerate cosmic ray protons up to 1e17 eV. These energetic protons lose a good fraction of their energy through proton-proton collision when ejected into very dense interstellar medium, and as a result, produce high energy neutrinos (, Comment: 5 pages, 1 figures, accepted for publication as a regular article in Physical Review D