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The standard external shock model in the thin-shell scenario predicts an onset bump in the early optical afterglow light curves of gamma-ray bursts (GRBs). We collect such a textbook-version light curve sample of $30$ GRBs, and derive the jet properties from our joint fit to their X-ray and optical afterglow light curves. It is found that the distributions of the isotropic initial Lorentz factors ($\Gamma_0$), the deceleration radii ($R_{\rm dec}$), and the magnetic field strength ($B_0$) are log-normal, but the distributions of the isotropic kinetic energy ($E_{\rm k, iso}$), medium density ($n_{0}$), and the magnetization parameter ($\sigma_{B}\equiv\epsilon_B/\epsilon_e$) are tentatively bimodal. A tight $R_{\rm dec}\mbox{-}B_{0}\mbox{-}\sigma_{B}$ relation is found. It infers a universal $\epsilon_e E_{\rm k,iso}$ among bursts, plausibly supporting the previous argument of a universal GRB radiation energy among GRBs. A jet break is required for modeling the light curves of $26$ GRBs. The distributions of the jet opening angles and the jet-corrected kinetic energies log-normally center at $\log \theta_{\rm j,c}/{\rm rad}=-1.51$ (standard deviation $\sigma=0.27$) and $\log (E_{\rm k, j,c}/{\rm erg})=51.78$ ($\sigma=0.54$), respectively. Those GRBs ($19$ GRBs), whose prompt gamma-ray emission is well estimated with broad energy-band observations, satisfy the previously discovered $L_{\rm \gamma, p, iso}-E_{\rm p,z}-\Gamma_{0}$ relation, and their gamma-ray radiation efficiencies log-normally distribute in the range from $0.04\%$ to $10\%$ with a central value of $0.42\%$. Such a low efficiency favors the baryonic fireball model, and the distribution of their baryon mass loading in the GRB ejecta log-normally centers at $\log (M_{\rm fb,c}/M_{\rm sun})=-5$ ($\sigma=0.75$)., Comment: 12 pages, 4 tables, 9 figures. Publication in the Astrophysics Journal (in press)

Recently, Beloborodov suggested that there exists a resonance phenomenon between an extremely intense electromagnetic wave and internal magnetized particles. The particles exchange energy with the wave at frequent resonance events and then reach the radiation reaction limit immediately. This process greatly enhances the scattering cross section of the particles. Note that these results only involve an extraordinary (X) mode wave. In this paper, we focus on an intense ordinary (O) mode wave propagating through magnetized particles and compare it with the case of the X-mode wave. Our result shows that the scattering cross section of the particles in the O-mode wave is significantly smaller than that in the X-mode wave. This has important implications for the transparency of a fast radio burst (FRB) inside the magnetosphere of a magnetar. We argue that there is a strong scattering region in the stellar magnetosphere, within which an O-mode wave is more transparent than an X-mode wave for an FRB., Comment: 8 pages, 5 figures, accepted by ApJ

AT2018cow is the most extensively observed and widely studied fast blue optical transient to date; its unique observational properties challenge all existing standard models. In this paper, we model the luminosity evolution of the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum of AT2018cow with a magnetar-centered engine model. We consider a two-zone model with a striped magnetar wind in the interior and an expanding ejecta outside. The soft and hard X-ray emission of AT2018cow can be explained by the leakage of high-energy photons produced by internal gradual magnetic dissipation in the striped magnetar wind, while the luminous thermal UV/optical emission results from the thermalization of the ejecta by the captured photons. The two-component energy spectrum yielded by our model with a quasi-thermal component from the optically thick region of the wind superimposed on an optically thin synchrotron component well reproduces the X-ray spectral shape of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in order to explain the very short rise time to peak of the thermal optical emission, a low ejecta mass $M_{\rm ej}\approx0.1~M_\odot$ and high ejecta velocity $v_{\rm SN}\approx0.17c$ are required. A millisecond magnetar with $P_0\approx3.7~\rm ms$ and $B_p\approx2.4\times10^{14}~\rm G$ is needed to serve as the central engine of AT2018cow., Comment: 9 Pages, 5 Figures, 1 Tables, Accepted by ApJL; https://doi.org/10.3847/2041-8213/ad2611

Very recently, a particularly long gamma-ray burst (GRB) 230307A was reported and proposed to originate from a compact binary merger based on its host galaxy property, kilonova, and heavy elements. More intriguingly, a very early plateau followed by a rapid decline in soft X-ray band was detected in its light curve by the Lobster Eye Imager for Astronomy, indicating strong evidence of the existence of a magnetar as the merger product. This work explores that the Magnetar Wind Internal Gradual MAgnetic Dissipation (MIGMAD) model, in which the radiative efficiency evolves over time, successfully fits it to the observed data. Our results reinforce the notion that the X-ray plateau serves as a powerful indicator of a magnetar and imply that an evolving efficiency is likely to be a common feature in X-ray plateaus of GRB afterglows. In addition, we also discuss the explanations for the prompt emission, GRB afterglows, as well as kilonova, and predict possible kilonova afterglows in a magnetar central engine., Comment: 10 pages, 4 figures, 2 tables. Accepted for publication in The Astrophysical Journal Letters

Fast radio bursts (FRBs) are millisecond-duration extragalactic radio transients. They apparently fall into repeaters and non-repeaters. However, such a classification has lacked a motivation on the physical picture. Here we propose a unified geometric model to distinguish between the repeaters and non-repeaters, in which the quasi-tangential (QT) propagation effect within the magnetospheric polar cap of a neutron star is considered. In this model, the non-repeaters arise from the sources whose emitting region has a smaller impact angle with respect to the magnetic axis, while the repeaters come from the sources whose emitting region has a larger impact angle. The observational discriminant polarization properties between the repeaters and non-repeaters are an important clue to verifying this unified geometric model since the polarization is sensitive to the QT propagation effect. Moreover, our model effectively explains all of the other discriminant properties, including bandwidth, duration, peak luminosity, energy, brightness temperature, time-frequency downward drifting, and repetition rate, providing compelling evidence for the magnetospheric origin of FRBs., Comment: 11 pages, 8 figures, published version

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., Comment: 16 Pages, 9 Figures, 2 Tables, Accepted by ApJL

The gamma-ray burst GRB 211211A and its associated kilonova-like emission were reported recently. A significant difference between this association event and GRB 170817A/AT 2017gfo is that GRB 211211A has a very long duration. In this paper, we show that this association event may arise from a neutron star$-$white dwarf (NS$-$WD) merger if a magnetar leaves finally in the central engine. Within the NS$-$WD merger, the main burst of GRB 211211A could be produced by magnetic bubble eruptions from toroidal magnetic field amplification of the pre-merger NS. This toroidal field amplification can be induced by the runaway accretion from the WD debris disc if the disc is in low initial entropy and efficient wind. While the extended emission of GRB 211211A is likely involved with magnetic propelling. The observed energetics and duration of the prompt emission of GRB 211211A can be fulfilled in comparison with those of accretion in hydrodynamical thermonuclear simulation, as long as the WD has a mass $\gtrsim1M_{\odot}$. Moreover, if the X-ray plateau in GRB afterglows is due to the magnetar spin-down radiation, GRB optical afterglows and kilonova-like emission can be well jointly modeled combining the standard forward shock with the radioactive decay power of $^{56}{\rm Ni}$ adding a rotational power input from the post-merger magnetar., Comment: Accepted by ApJ Letters. The accretion rate can be >0.01$M_{\odot}s^{-1}$ if the WD debris disc is in low initial entropy simulated by Kaltenborn et al. (2022). Main burst vs. extended emission: accretion vs. propelling. Kilonova-like emission: magnetar-fed $^{56}{\rm Ni}$ power

The physical origin of fast radio bursts (FRBs) remains unclear. Finding multiwavelength counterparts of FRBs can be a breakthrough in understanding their nature. In this work, we perform a systematic search for astronomical transients (ATs) whose positions are consistent with FRBs. We find an unclassified optical transient AT2020hur ($\alpha=01^{\mathrm{h}} 58^{\mathrm{m}} 00.750^{\mathrm{s}} \pm 1$ arcsec, $\delta=65^{\circ} 43^{\prime} 00.30^{\prime \prime} \pm 1$ arcsec) that is spatially coincident with the repeating FRB 180916B ($\alpha=01^{\mathrm{h}} 58^{\mathrm{m}} 00.7502^{\mathrm{s}} \pm 2.3$ mas, $\delta=65^{\circ} 43^{\prime} 00.3152^{\prime \prime} \pm 2.3$ mas). The chance possibility for the AT2020hur-FRB 180916B association is about 0.04%, which corresponds to a significance of $3.5\sigma$. We develop a giant flare afterglow model to fit AT2020hur. Although the giant flare afterglow model can interpret the observations of AT2020hur, the derived kinetic energy of such a GF is at least three orders of magnitude larger than that of the typical GF, and there is a lot of fine tuning and coincidences required for this model. Another possible explanation is that AT2020hur might consist of two or more optical flares originating from the FRB source, e.g. fast optical bursts produced by the inverse Compton scattering of FRB emission. Besides, AT2020hur is located in one of the activity windows of FRB 180916B, which is an independent support for the association. This coincidence may be due to the reason that the optical counterpart is subject to the same periodic modulation as FRB 180916B, as implied by the prompt FRB counterparts. Future simultaneous observations of FRBs and their optical counterparts may help reveal their physical origin., Comment: 17 pages, 6 figures, accepted for publication in ApJ

To probe this question, we perform a statistical analysis using the first Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) catalog and identify a few discriminant properties between repeating and non-repeating FRBs such as the repetition rate, duration, bandwidth, spectral index, peak luminosity, and potential peak frequency. If repeating and non-repeating FRBs belong to one population, their distribution distinctions for the repetition rate and duration can be explained by the selection effect due to the beamed emission as in Connor et al. (2020). However, we obtain that the distribution distinctions for the spectral index and potentially the peak frequency cannot be explained by the beamed emission within the framework of either the coherent curvature radiation or synchrotron maser emission. This indicates that there could be two populations. We further discuss three possible scenarios for the required two populations., Comment: The answer is "maybe no". ApJ in press

Assuming that the shallow-decaying phase in the early X-ray lightcurves of gamma-ray bursts (GRBs) is attributed to the dipole radiations (DRs) of a newborn magnetar, we present a comparative analysis for the magnetars born in death of massive stars and merger of compact binaries with long and short GRB (lGRB and sGRB) data observed with the {\em Swift} mission. We show that the typical braking index ($n$) of the magnetars is $\sim 3$ in the sGRB sample, and it is $\sim 4$ for the magnetars in the lGRB sample. Selecting a sub-sample of the magnetars whose spin-down is dominated by DRs ($n\lesssim 3$) and adopting a universal radiation efficiency of $0.3$, we find that the typical magnetic field strength ($B_p$) is $10^{16}$ G {\em vs.} $10^{15}$ G and the typical initial period ($P_0$) is $\sim 20$ ms {\em vs.} $2$ ms for the magnetars in the sGRBs {\em vs.} lGRBs. They follow the same relation between $P_0$ and the isotropic GRB energy as $ P_0\propto E_{\rm jet}^{-0.4}$. We also extend our comparison analysis to superluminous supernovae (SLSNe) and stable pulsars. Our results show that a magnetar born in merger of compact stars tends to have a stronger $B_p$ and a longer $P_0$ by about one order of magnitude than that born in collapse of massive stars. Its spin-down is dominated by the magnetic DRs as old pulsars, being due to its strong magnetic field strength, whereas the early spin-down of magnetars born in massive star collapse is governed by both the DRs and gravitational wave (GW) emission. A magnetar with a faster rotation speed should power a more energetic jet, being independent of its formation approach., Comment: 10 pages, 2 tables, 7 figures. Published in MNRAS

In this work, we propose an accreting stellar binary model for understanding the active periodic fast radio bursts (FRBs). The system consists of a stellar compact object (CO) and a donor star (DS) companion in an eccentric orbit, where the DS fills its own Roche lobe near the periastron. The CO accretes the material from the DS and then drive relativistic magnetic blobs. The interaction between the magnetic blobs and the stellar wind of the DS produces a pair of shocks. We find that both of the reverse shock and the forward shock are likely to produce FRBs via synchrotron maser mechanism. We show that this system can in principle sufficiently produce highly active FRBs with a long lifetime, and also can naturally explain the periodicity and the duty cycle of the activity as appeared in FRBs 180916 and 121102. The radio nebula excited by the long-term injection of magnetic blobs into the surrounding environment may account for the associated persistent radio source. In addiction, we discuss the possible multi-wavelength counterparts of FRB 180916 in the context of this model. Finally, we encourage the search for FRBs in the ultraluminous X-ray sources., Comment: 11 pages, 2 figures; accepted to APJ

Soft gamma-ray repeaters (SGRs) are mainly a Galactic population and originate from neutron stars with intense ($B\simeq 10^{15}{\rm \ G}$) magnetic fields ('magnetars'). Occasionally, a giant flare occurs with enormous intensity, displaying a short hard spike, followed by a weaker, oscillatory phase which exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), rasing the question whether it is a classic short GRB or a magentar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in $1\,{\rm keV} - 10\,{\rm MeV}$, i.e., $E_{\rm p}\propto E_{\rm iso}^{1/4}$, and locate in a distinct region of the $E_{\rm p}-E_{\rm iso}$ plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireball. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favor the giant flare origin of these GRBs. The GeV emission detected by \emph{Fermi}/LAT from GRB 200415A at $18-285 \,$s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of $\sim 10^{23}{\rm g}$ in the giant flare fireball of GRB 200415A., Comment: Accepted by ApJL, 8 pages, 4 figures

In this paper, we investigate the energy-source models for the most luminous supernova ASASSN-15lh. We revisit the ejecta-circumstellar medium (CSM) interaction (CSI) model and the CSI plus magnetar spin-down with full gamma-ray/X-ray trapping which were adopted by \cite{Chatzopoulos16} and find that the two models cannot fit the bolometric LC of ASASSN-15lh. Therefore, we consider a CSI plus magnetar model with the gamma-rays/X-rays leakage effect to eliminate the late-time excess of the theoretical LC. We find that this revised model can reproduce the bolometric LC of ASASSN-15lh. Moreover, we construct a new hybrid model (i.e., the CSI plus fallback model), and find that it can also reproduce the bolometric LC of ASASSN-15lh. Assuming that the conversion efficiency ($\eta$) of fallback accretion to the outflow is typically $\sim10^{-3}$, we derive that the total mass accreted is $\sim3.9~M_\odot$. The inferred CSM mass in the two models is rather large, indicating that the progenitor could have experienced an eruption of hydrogen-poor materials followed by an energetic core-collapse explosion leaving behind a magnetar or a black hole., Comment: 13 pages including 3 tables and 5 figures, accepted for publication in ApJ

It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star--white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by \cite{mar19}. By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies., Comment: Published in ApJ on 8 April 2020

The LIGO-Virgo and Fermi collaborations recently reported a possible joint detection of a sub-threshold gravitational wave (GW) event and a sub-threshold gamma-ray burst (GRB), GBM-190816, that occurred 1.57 s after the merger. We perform an independent analysis of the publicly available data and investigate the physical implications of this potential association. By carefully studying the following properties of GBM-190816 using Fermi/GBM data, including signal-to-noise ratio, duration, f-parameter, spectral properties, energetic properties, and its compliance with some GRB statistical correlations, we confirm that this event is likely a typical short GRB. Assuming its association with the sub-threshold GW event, the inferred luminosity is $1.47_{-1.04}^{+3.40} \times 10^{49}$ erg s$^{-1}$. Based on the available information of the sub-threshold GW event, we infer the mass ratio q of the compact binary as $q=2.26_{-1.43}^{+2.75}$ according to the reported range of luminosity distance. If the heavier compact object has a mass > 3 solar masses, q can be further constrained to $q=2.26_{-0.12}^{+2.75}$. The leading physical scenario invokes an NS-BH merger system with the NS tidally disrupted. Within this scenario, we constrain the physical properties of such a system to produce a GRB. The GW data may also allow an NS-BH system with no tidal disruption of the NS or a BH-BH merger. We apply the charged compact binary coalescence (cCBC) theory (for both a constant charge and an increasing charge for the merging members) to derive the model parameters to account for GBM-190816 and found that the required parameters are extreme. Finally, we argue that the fact that the observed GW-GRB delay time scale is comparable to that of GW170817/GRB 170817A suggests that the GW-GRB time delay of these two cases is mainly defined by the time scale for the jet to propagate to the GRB emission site., Comment: 23 pages, 12 figures. Accepted by ApJ

Two accretion columns have been argued to form over the surface of a newborn millisecond magnetar for an extremely high accretion rate $\gtrsim1.8\times10^{-2}M_\odot\ {\rm s^{-1}}$ that may occur in the core-collapse of a massive star. In this paper, we investigate the characteristics of these accretion columns and their gravitational wave (GW) radiation. For a typical millisecond magnetar (surface magnetic field strength $B\sim10^{15}$ G and initial spin period $P\sim1$ ms), we find (1) its accretion columns are cooled via neutrinos and can reach a height $\sim1$ km over the stellar surface; (2) its column-induced characteristic GW strain is comparable to the sensitivities of the next generation ground-based GW detectors within a horizon $\sim1$ Mpc; (3) the magnetar can survive only a few tens of seconds; (4) during the survival timescale, the height of the accretion columns increases rapidly to the peak and subsequently decreases slowly; (5) the column mass, characteristic GW strain, and maximum GW luminosity have simultaneous peaks in a similar rise-fall evolution. In addition, we find that the magnetar's spin evolution is dominated by the column accretion torque. A possible association with failed supernova is also discussed., Comment: 8 pages, 3 figures, accepted for publication in PRD

We extract 18 candidate short gamma-ray bursts (SGRBs) with precursors from 660 SGRBs observed by {\em Fermi} and {\em Swift} satellites, and carry out a comprehensive analysis on their temporal and spectral features. We obtain the following results: (1) For a large fraction of candidates, the main burst durations are longer than their precursor durations, comparable to their quiescent times from the end of precursors to the beginning of their main bursts. (2) The average flux of precursors tends to increase as their main bursts brighten. (3) As seen from the distributions of hardness ratio and spectral fitting, the precursors are slightly spectrally softer with respect to the main bursts. Moreover, a significant portion of precursors and all main bursts favor a non-thermal spectrum. (4) The precursors might be a probe of the progenitor properties of SGRBs such as the magnetic field strength and the crustal equation of state if they arise from some processes before mergers of binary compact objects rather than post-merger processes., Comment: 28 pages, 6 figures including 48 panels, 2 tables, accepted for publication in The Astrophysical Journal

For a binary composed of a spinning black hole (BH) (with mass $\gtrsim 7M_\odot$) and a strongly magnetized neutron star (NS) (with surface magnetic field strength $B_{\rm S,NS}\gtrsim10^{12}$\,G and mass $\sim 1.4M_\odot$), the NS as a whole will possibly eventually plunge into the BH. During the inspiral phase, the spinning BH could be charged to the Wald charge quantity $Q_{\rm W}$ until merger in an electro-vacuum approximation. During the merger, if the spinning charged BH creates its own magnetosphere due to an electric field strong enough for pair cascades to spark, the charged BH would transit from electro-vacuum to force-free cases and could discharge in a time $\gtrsim1~{\rm ms}$. As the force-free magnetosphere is full of a highly conducting plasma, the magnetic flux over the NS's caps would be retained outside the BH's event horizon under the frozen-in condition. Based on this scenario, we here investigate three possible energy-dissipation mechanisms that could produce electromagnetic (EM) counterparts in a time interval of the BH's discharge post a BH-NS merger-induced gravitational wave event: (1) magnetic reconnection at the BH's poles would occur, leading to a millisecond bright EM signal, (2) a magnetic shock in the zone of closed magnetic field lines due to the detachment and reconnection of the entire BH magnetic field would probably produce a bright radio emission, e.g., a fast radio burst, and (3) the Blandford-Znajek mechanism would extract the BH's rotational energy, giving rise to a millisecond-duration luminous high-energy burst. We also calculate the luminosities due to these mechanisms as a function of BH's spin for different values of $B_{\rm S,NS}$., Comment: 8 pages, 4 figures, accepted for publication in ApJ Letters

Gamma-ray burst (GRB) central engines and jet production mechanisms are still open questions. Assuming that the shallow decay segments of canonical X-ray afterglow lightcurves of {\em Swift} GRBs are attributed to the magnetic dipole (MD) radiations of newly-born magnetars, we derive the parameters of the magnetars and explore their possible relations to jet and MD wind emission. We show that the magnetar initial spin period ($P_0$) are tightly correlated with the jet energy ($E_{\rm jet}$), which is almost proportional to the wind energy ($E_{\rm wind}$). Our least square fits yield $P_0\propto E^{-0.36\pm 0.03}_{\rm jet}$ and $E_{\rm wind}\propto E^{0.91\pm 0.07}_{\rm jet}$. These relations may imply that a magnetar with faster rotating speed can power a more energetic GRB, and energy partition between the jet and wind may be quasi-universal. Although the $P_0-E_{\rm jet}$ relation is driven by a few sub-energetic GRBs in our sample, our Monte Carlo simulation analysis shows that sample selection biases from instrumental flux limits and contaminations of the bright jet afterglows cannot make this correlation. Within this jet-wind paradigm, we propose that GRB 101225A-like ultra-long GRBs, whose prompt gamma-ray/X-ray lightcurves are featured as a long-lasting plateau with a sharp drop, may be the orphan MD wind emission being due to misalignment of their jet axis to the light of sight. Brief discussion on the orphan MD wind emission and its association with the gravitational wave radiation of newly-born magnetars is presented., Comment: 19 pages, 2 tables, 12 figures, accepted for publication in ApJ

Two-episode emission components separated by quiescent gaps in the prompt emission of gamma-ray bursts (GRBs) have been observed in the {\em Swift} era, but there is a lack of spectral information due to the narrow energy band of the {\em Swift}/Burst Alert Telescope. In this paper, a systematic analysis of the spectral and temporal properties of the prompt emission of 101 {\em Fermi}/Gamma-ray Burst Monitor detected long GRBs show the existence of two-episode emission components in the light curves, with quiescent times of up to hundreds of seconds. We focus on investigating the differences of those two emission episodes. We find that the light curves of the two emission components exhibit different behavior, e.g., a soft emission component that either precedes or follows the main prompt emission or that the intensity of the two emission episodes are comparable with each other. No statistically significant correlation in the duration of the two emission episodes can be claimed. We define a new parameter $\varepsilon$ as the ratio of the peak flux of the first and second emission episodes and find that a higher $\varepsilon$ corresponds to a larger fluence. The preferred spectral model in our analysis is a cutoff power-law model for most GRBs. The distribution of $E_p$ for episodes I and II range from tens of keV to 1000 keV with a lognormal fit and there are no significant differences between them. Moreover, we do not find significant relationships between $\varepsilon$ and $E_p$ for the two emission episodes. Those results suggest that these two-episode emission components likely share the same physical origin., Comment: 22 pages, 1 Table, 5 Figures. updated the reference list and matched the published version

We present our optical photometric and spectroscopical observations of GRB 140629A. A redshift of $z=2.275\pm0.043$ is measured through the metal absorption lines in our spectroscopic data. Using our photometric data and multiple observational data observed with other telescopes, we show that its optical light curve is well interpreted with the standard forward shock models in the thin shell case. Its optical-X-ray afterglow spectrum is jointly fitted with a single power-law function, yielding a photon index of $-1.90\pm 0.05$. The optical extinction and neutral hydrogen absorption of the GRB host galaxy are negligible. The fit to the light curve with the standard models shows that the ambient density is $60\pm 9$ cm$^{-3}$ and the GRB radiating efficiency is as low as $\sim 0.24\%$, likely indicating a baryonic-dominated ejecta of this GRB. This burst agrees well with the $L_{\rm p, iso}-E_p^{'}-\Gamma_0$ relation, but confidently violates those empirical relations involving geometric corrections (or jet break time). This gives rise to an issue of possible selection effect on these relations since the jet opening angle of this GRB is extremely narrow (0.04 rad)., Comment: ApJ accepted, 6 figures, 3 tables

GRB 160625B is an extremely bright GRB with three distinct emission episodes. By analyzing its data observed with the GBM and LAT on board the {\em Fermi} mission, we find that a multi-color black body (mBB) model can be used to fit the spectra of initial short episode (Episode I) very well within the hypothesis of photosphere emission of a fireball model. The time-resolved spectra of its main episode (Episode II), which was detected with both GBM and LAT after a long quiet stage ($\sim 180$ seconds) of the initial episode, can be fitted with a model composing of an mBB component plus a cutoff power-law (CPL) component. This GRB was detected again in the GBM and LAT bands with a long extended emission (Episode III) after a quiet period of $\sim 300$ seconds. The spectrum of Episode III is adequately fitted with a CPL plus a single power-law models, and no mBB component is required. These features may imply that the emission of three episodes are dominated by distinct physics process, i.e., Episode I is possible from cocoon emission surrounding the relativistic jet, Episode II may be from photosphere emission and internal shock of relativistic jet, and Episode III is contributed by internal and external shocks of relativistic jet. On the other hand, both X-ray and optical afterglows are consistent with standard external shocks model., Comment: 28 pages, 14 figures, and 2 Tables. Accepted for publication in ApJ

GRB 160821B is a short gamma-ray burst (GRB) at redshift $z=0.16$, with a duration less than 1 second and without detection of any "extended emission" up to more than 100 seconds in both {\em Swift}/BAT and {\em Fermi}/GBM bands. An X-ray plateau with a sharp drop 180 seconds after the BAT trigger was observed with {\em Swift}/XRT. No supernova or kilo-nova signature was detected. Assuming the central engine of this SGRB is a recently born supra-massive magnetar, we can explain the SGRB as jet radiation and its X-ray plateau as the internal energy dissipation of the pulsar wind as it spins down. We constrain its surface magnetic field as $B_{\rm p}<3.12\times 10^{16}$ G and initial spin period as $P_0< 8.5\times 10^{-3}$ seconds. Its equation of state is consistent with the GM1 model with $M_{\rm TOV} \sim 2.37 M_\odot$ and ellipticity $\epsilon<0.07$. Its gravitational wave (GW) radiation may be detectable with the future Einstein Telescope, but is much weaker than the current detectability limit of advanced-LIGO. The GW radiation of such an event would be detectable by advanced-LIGO if it occurred at a distance of 100 Mpc ($z=0.023$)., Comment: 10 pages, 7 figures, Accepted for publication in the ApJ

We report our observations of very bright prompt optical and reverse shock (RS) optical emission of GRB 140512A and analyze its multi-wavelength data observed with the {\em Swift} and {\em Fermi} missions. It is found that the joint optical-X-ray-gamma-ray spectrum with our first optical detection (R=13.09 mag) at $T_0+136$ seconds during the second episode of the prompt gamma-rays can be fit by a single power-law with index $-1.32\pm 0.01$. Our empirical fit to the afterglow lightcurves indicates that the observed bright optical afterglow with R=13.00 mag at the peak time is consistent with predictions of the RS and forward shock (FS) emission of external shock models. Joint optical-X-ray afterglow spectrum is well fit with an absorbed single power-law, with an index evolving with time from $-1.86\pm 0.01$ at the peak time to $-1.57\pm 0.01$ at late epoch, which could be due to the evolution of the ratio of the RS to FS emission fluxes. We fit the lightcurves with standard external models, and derive the physical properties of the outflow. It is found that the ratio $R_B\equiv\epsilon_{\rm B, r}/\epsilon_{\rm B, f}$ is 8187, indicating a high magnetization degree in the RS region. Measuring the relative radiation efficiency with $R_e\equiv\epsilon_{\rm e, r}/\epsilon_{\rm e, f}$, we have $R_e= 0.02$, implying the radiation efficiency of the RS is much lower than that in FS. We also show that the $R_B$ of GRBs 990123, 090102, and 130427A are similar to that of GRB 140512A and their apparent difference may be mainly attributed to the difference of the jet kinetic energy, initial Lorentz factor, and medium density among them., Comment: 11 pages in emulateapj style, 5 figures, 4 tables. Accepted for publication in ApJ

We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow lightcurve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band lightcurve. Our joint spectral fits of the optical and X-ray data show that they are in the same regime, with a photon index of $\sim 1.70$. The optical and X-ray afterglow lightcurves are well fitted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we find that the radiative efficiency of the GRB jet is $\sim 1\%$ and the magnetization parameter of the afterglow jet is $<0.04$ with the derived extremely low $\epsilon_B$ (the fraction of shock energy to magnetic field) of $(1.64\pm 0.25)\times 10^{-6}$. These results indicate that the jet may be matter dominated. Discussion on delayed energy injection from accretion of late fall-back material of its pre-supernova star is also presented., Comment: 24 pages, 5 figures, 3 tables, accpeted for publication in ApJ

A rapidly spinning, strongly magnetized neutron star is invoked as the central engine for some Gamma-ray bursts (GRBs), especially, the $"$internal plateau$"$ feature of X-ray afterglow. However, for these $"$internal plateau$"$ GRBs, how to produce their prompt emission remains an open question. Two different physical process have been proposed in the literature, (1) a new-born neutron star is surrounded by a hyper-accreting and neutrino cooling disk, the GRB jet can be powered by neutrino annihilation aligning the spin axis; (2) a differentially rotating millisecond pulsar was formed due to different angular velocity between the interior core and outer shell parts of the neutron star, which can power an episodic GRB jet. In this paper, by analyzing the data of one peculiar GRB 070110 (with internal plateau), we try to test which model being favored. By deriving the physical parameters of magnetar with observational data, the parameter regime for initial period ($P_{0\rm }$) and surface polar cap magnetic field ($B_{\rm p}$) of the central NS are $(0.96\sim 1.2 )~\rm ms$ and $(2.4\sim 3.7)\times 10^{14}~\rm G$, respectively. The radiative efficiency of prompt emission is about $\eta_{\gamma} \sim 6\%$. However, the radiative efficiency of internal plateau ($\eta_{\rm X}$) is larger than $31\%$ assuming the $M_{\rm NS}\sim1.4 M_{\odot}$ and $P_{0\rm }\sim1.2 ~\rm ms$. The clear difference between the radiation efficiencies of prompt emission and internal plateau implies that they maybe originated from different components (e.g. prompt emission from the relativistic jet powered by neutrino annihilation, while the internal plateau from the magnetic outflow wind)., Comment: 5 pages, 2 figures. 1 Table, Accepted for publication in MNRAS

Observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (GRBs). We report our optical observations of GRB 111228A from 95 sec to about 50 hours after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data observed with {\em Swift} and {\em Fermi} missions. Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features as low dust-to-gas ratio. Incorporating the energy injection effect, our best fit to the afterglow lightcurves with the standard afterglow model via the Markov Chain Monte Carlo (MCMC) technique shows that $\epsilon_e=(6.9\pm 0.3)\times 10^{-2}$, $\epsilon_B=(7.73\pm 0.62)\times 10^{-6}$, $E_{\rm K}=(6.32\pm 0.86)\times 10^{53}\rm erg$, $n=0.100\pm 0.014$ cm$^{-3}$. The low medium density likely implies that the afterglow jet may be in a halo or in a hot ISM. Achromatic shallow decay segment observed in the optical and X-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. The $E_p$ of its time-integrated prompt gamma-ray spectrum is $\sim 26$ KeV. Using the initial Lorentz factor ($\Gamma_0=476^{+225}_{-237}$) derived from our afterglow model fit, it is found that GRB 111228A satisfies the $L_{\rm iso}-E_{\rm p,z}-\Gamma_0$ relation and bridges the typical GRBs and low luminosity GRBs in this relation., Comment: 33 pages, 18 figures, 2 tables. Accepted by ApJ