Search

Fast radio bursts (FRBs) are mysterious radio transients with millisecond durations. Recently, a periodic activity of $\sim$16 day and a possible periodicity of $\sim$159 day were detected to arise from FRB 180916.J0158+65 and FRB 121102, respectively, and the spin period of a slow-rotation magnetar was further considered to be one of possibilities to explain the periodic activities of repeating FRBs. For isolated neutron stars, the spin evolution suggests that it's difficult to reach several hours. In this work, we mainly focus on the possible maximum spin period of isolated NSs / magnetars dominated by an interaction between star's magnetic field and the disk. We find that the disk wind plays an important role in spin evolution, whose influence varies the power law index in the evolution equation of mass flow rate. For a magnetar without disk wind, the longest spin period is tens of hours. When the disk wind with a classical parameter is involved, the maximum spin period can reach hundreds of hours. But for a much extremely large index of mass flow rate due to disk wind or other angular momentum extraction processes, a spin period of $\sim$(16-160) days is still possible., Comment: Accepted for publication in ApJ

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

Very recently a fast radio burst (FRB) 200428 associated with a strong X-ray burst from the Galactic magnetar SGR 1935+2154 has been detected, which is direct evidence supporting the magnetar progenitor models of FRBs. Assuming the FRB radiation mechanism is synchrotron maser emission from magnetized shocks, we develop a specific scenario by introducing a density jump structure of upstream medium, and thus the double-peaked character of FRB 200428 is a natural outcome. The luminosity and emission frequency of two pulses can be well explained in this scenario. Furthermore, we find that the synchrotron emission of shock-accelerated electrons is in the X-ray band, which therefore can be responsible for at least a portion of observed X-ray fluence. With proper upgrade, this density jump scenario can be potentially applied to FRBs with multiple peaks in the future., Comment: 6 pages, 2 figures

Fast radio bursts (FRBs) are a new kind of extragalactic radio transients. Some of them show repeating behaviors. Recent observations indicate that a few repeating FRBs (e.g., FRB 121102) present time--frequency downward drifting patterns and nearly 100$\%$ linear polarization. Following the model of \citet{dai 2016} who proposed that repeating FRBs may originate from a slowly-rotating, old-aged pulsar colliding with an asteroid belt around a stellar-mass object, we focus on the prediction of time--frequency drifting and polarization. In this scenario, the frequency drifting is mainly caused by the geometric structure of a pulsar magnetosphere, and the drifting rate--frequency index is found to be $25/17$. On the other hand, by considering the typical differential mass distribution of incident asteroids, we find that an asteroid with mass $m\gtrsim 10^{17}~{\rm g}$ colliding with the pulsar would contribute abundant gravitational energy, which powers an FRB. A broad frequency band of the FRBs would be expected, due to the mass difference of the incident asteroids. In addition, we simulate the linear polarization distribution for the repeating FRBs, and constrain the linear polarization with $\gtrsim$ 30$\%$ for the FRBs with flux of an order of magnitude lower than the maximum flux., Comment: 9 pages, 6 figures, accepted for publication in ApJ

Accompanied by an X-ray burst, the fast radio burst (FRB) FRB 200428 was recently confirmed as originating from the Galactic magnetar soft gamma repeater (SGR) SGR J1935+2154. Just before and after FRB 200428 was detected, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) had been monitoring SGR J1935+2154 for eight hours. From UTC 2020 April 27 23:55:00 to 2020 April 28 00:50:37, FAST detected no pulsed radio emission from SGR J1935+2154, while Fermi/Gamma-ray Burst Monitor registered 34 bursts in the X/soft $\gamma$-ray band, forming a unique sample of X-ray bursts in the absence of FRBs. After a comprehensive analysis on light curves, time-integrated, and time-resolved spectral properties of these FRB-absent X-ray bursts, we compare this sample with the FRB-associated X-ray burst detected by Insight-HXMT, INTEGRAL, and Konus-Wind. The FRB-associated burst distinguishes itself from other X-ray bursts by its nonthermal spectrum and a higher spectral peak energy, but otherwise is not atypical. We also compare the cumulative energy distribution of our X-ray burst sample with that of first repeating FRB source, FRB 121102, with the calibration of FRB 200428-X-ray burst association. We find a similarity between the two, offering indirect support of the magnetar origin of cosmological FRBs. The event rate density of magnetar bursts is about $\sim 150$ times higher than the FRB event rate density at the energy of FRB 200428. This again suggests that, if all FRBs originate from magnetars, only a small fraction of X-ray bursts are associated with FRBs., Comment: 18 pages, 3 tables, 8 figures. Published in the Astrophysical Journal Letter on January 12, 2021

Synchrotron emission polarization is very sensitive to the magnetic field configuration. Recently, polarization of synchrotron emission with a mixed (SM) magnetic field in Gamma-ray burst (GRB) afterglow phase had been developed. Here, we apply these SM models to GRB prompt phase and compare their polarization properties with that of synchrotron emission in purely ordered (SO) magnetic field. We find that the polarization properties in a SM model are very similar to these in a corresponding SO model (e.g., synchrotron emission in a mixed magnetic field with an aligned ordered part (SMA) and synchrotron emission with a purely ordered aligned magnetic field (SOA)), only with a lower polarization degree (PD). We also discuss the statistical properties of the models. We find PDs of the simulated bursts are concentrated around $25\%$ for both SOA and synchrotron emission in a purely ordered toroidal magnetic field (SOT), while they can range from $0\%$ to $25\%$ for SMA and synchrotron emission in a mixed magnetic field with a toroidal ordered part (SMT), depending on $\xi_B$ value, i.e., the ratio of magnetic reduction of the ordered magnetic field over that of random magnetic field. From statistics, if PDs of majority GRBs are non-zero, then it favours SO and SM models. Further, if there are some bright GRBs with a prominently lower PDs than that of the majority GRBs, it favours SOT (SMT) models; if all the bright GRBs have comparable PDs with the majority ones, it favours SOA (SMA) models. Finally, we apply our results to POLAR's data and find that $\sim10\%$ time-integrated PDs of the observed bursts favor SMA and SMT models, and $\xi_B$ parameter of these bursts is constrained to be around 1.135., Comment: 23 pages, 10 figures, accepted by RAA

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

The association of FRB 200428 with an X-ray burst (XRB) from the Galactic magnetar SGR 1935+2154 offers important implications for the physical processes responsible for the fast radio burst (FRB) phenomena. By assuming that the XRB emission is produced in the magnetosphere, we investigate the possibility that the FRB emission is produced by shock-powered synchrotron maser (SM), which is phenomenologically described with a plenty of free parameters. The observational constraints on the model parameters indicate that the model can in principle be consistent with the FRB 200428 observations, if the ejecta lunched by magnetar activities can have appropriate ingredients and structures and the shock processes occur on the line of sight. To be specific, a complete burst ejecta should consist of an ultra-relativistic and extremely highly collimated $e^{\pm}$ component and a sub-relativistic and wide-spreading baryonic component. The internal shocks producing the FRB emission arise from a collision between the $e^{\pm}$ ejecta and the remnant of a previous baryonic ejecta at the same direction. The parameter constraints are still dependent on the uncertain spectrum and efficiency of the SM emission. While the spectrum is tentatively described by a spectral index of $-2$, we estimate the emission efficiency to be around $10^{-4}$ by requiring that the synchrotron emission of the shocked material cannot be much brighter than the magnetosphere XRB emission., Comment: 7 pages, 1 figure, 1 table

The first repeating fast radio burst (FRB), FRB 121102, was found to be associated with a spatially coincident, persistent nonthermal radio source, but the origin of the persistent emission remains unknown. In this paper, we propose that the persistent emission is produced via synchrotron-heating process by multiple bursts of FRB 121102 in a self-absorbed synchrotron nebula. As a population of bursts of the repeating FRB absorbed by the synchrotron nebula, the energy distribution of electrons in the nebula will change significantly. As a result, the spectrum of the nebula will show a hump steadily. For the persistent emission of FRB 121102, the total energy of bursts injecting into the nebula is required to be about $3.3\times10^{49}\,\unit{erg}$, the burst injection age is over $6.7\times 10^4\,\unit{yr}$, the nebula size is $\sim0.02\,\unit{pc}$, and the electron number is about $3.2\times10^{55}$. We predict that as more bursts inject, the brightness of the nebula would be brighter than the current observation, and meanwhile, the peak frequency would become higher. Due to the synchrotron absorption of the nebula, some low-frequency bursts would be absorbed, which may explain why most bursts were detected above $\sim1~\unit{GHz}$., Comment: 11 pages, 9 figures, accepted for publication in ApJ

The reverse shock (RS) model is generally introduced to interpret the optical afterglows with the rapid rising and decaying, such as the early optical afterglow of GRB 990123 (which is also called optical flash). In this paper, we collected 11 gamma-ray bursts (GRBs) early optical afterglows, which have such signatures of dominant RS emission. Since the temporal slopes of the optical flashes are determined by both the medium density distribution index $k$ and the electron spectral index $p$, we apply the RS model of the thin shell case to the optical flashes and determine the ambient medium of the progenitors. We find that the $k$ value is in the range of 0 - 1.5. The $k$ value in this paper is consistent with the result in Yi et al. (2013), where the forward shock (FS) model was applied to some onset bumps. However, the method adopted in this paper is only applicable to GRB afterglows with significant sharp rising and decaying RS emission. Our results indicate that the RS model can also be applied to confirm the circumburst medium, further implying that GRBs may have diverse circumburst media., Comment: Accepted for publication in The Astronomical Journal; 10 pages, 4 figures and 2 tables

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

Besides light curves and spectra, polarization provides a different powerful tool of studying the $\gamma-$ray burst (GRB) prompt phase. Compared with the time-integrated and energy-integrated polarization, time-resolved and energy-resolved polarization can deliver more physical information about the emitting region. Here we investigate time-resolved and energy-resolved polarization of GRB prompt emission using the synchrotron models. We find that the equal arrival time surface effect is very important in shaping the PD curves when the physical conditions of emitting region changes violently with radius. Polarization properties are neither correlated with the spectral lag nor the peak energy evolution patterns. Polarization properties with a mixed magnetic field are very similar to those for a corresponding ordered magnetic field but the former has a smaller polarization degree. The emission at the MeV peak can be highly polarized for a synchrotron model while it is unpolarized as predicted by a dissipative photosphere model. Future energy-resolved polarization observations can distinguish between these two models., Comment: 20 pages, 6 figures, Accepted by ApJ

In this paper, we present the study for the energy reservoir powering the light curves (LCs) of PS1-12cil and SN 2012aa which are superluminous and luminous supernovae (SNe), respectively. The multi-band and bolometric LCs of these two SNe show unusual secondary bumps after the main peaks. The two-peaked LCs cannot be explained by any simple energy-source models (e.g., the $^{56}$Ni cascade decay model, the magnetar spin-down model, and the ejecta-circumstellar medium interaction model). Therefore, we employ the $^{56}$Ni plus ejecta-circumstellar medium (CSM) interaction (CSI) model, the magnetar plus CSI model, and the double CSI model to fit their bolometric LCs, and find that both these two SNe can be explained by the double CSI model and the magnetar plus CSI model. Based on the modeling, we calculate the the time when the shells were expelled by the progenitors: provided that they were powered by double ejecta-shell CSI, the inner and outer shells might be expelled $\sim 0.2-3.6$ and $\sim 2-25$ years before the explosions of the SNe, respectively; the shells were expelled $\sim 2-20$ years before the explosions of the SNe if they were powered by magnetars plus CSI., Comment: 11 pages, 3 figures, 3 tables, accepted for publication in ApJ

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

The properties of fast radio bursts (FRBs) indicate that the physical origin of this type of astrophysical phenomenon is related to neutron stars. The first detected repeating source, FRB 121102, is associated with a persistent radio counterpart. In this paper, we propose that this radio counterpart could arise from a pulsar wind nebula powered by a magnetar without surrounding supernova ejecta. Its medium is a stratified structure produced by a progenitor wind. The model parameters are constrained by the spectrum of the counterpart emission, the size of the nebula, and the large but decreasing rotation measure (RM) of the repeating bursts. In addition, the observed dispersion measure is consistent with the assumption that all of the RM comes from the shocked medium., Comment: 10 pages, 4 figures, 1 table. Published in the Astrophysical Journal on Nov. 11, 2019

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

It was recently proposed that a significant fraction of ultraluminous X-ray sources (ULXs) actually host a neutron star (NS) accretor. We have performed a systematic study on the NS ULX population in Milky Way-like galaxies, by combining binary population synthesis and detailed stellar evolution calculations. Besides a normal star, the ULX donor can be a helium star (the hydrogen envelope of its progenitor star was stripped during previous common envelope evolution) if the NS is accreting at a super-Eddington rate via Roche lobe overflow. We find that the NS$-$helium star binaries can significantly contribute the ULX population, with the overall number of about several in a Milky Way-like galaxy. Our calculations show that such ULXs are generally close systems with orbital period distribution peaked at $ \sim 0.1 $ day (with a tail up to $ \sim100 $ days), and the helium stars have relatively low masses distributing with a maximum probability at $ \sim 1M_{\odot} $., Comment: 9 pages, 7 figures, accepted by ApJ

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

A central compact object (CCO, e.g. a black hole) with an accretion disk has been suggested as the common central engine of various astrophysical phenomena, such as gamma-ray bursts (GRBs), tidal disruption events (TDEs) and active galactic nuclei (AGNs). A jet powered by such a system might precess due to the misalignment of the angular momenta of the CCO and accretion disk. Some quasi-periodic behaviors observed in the light curves of these phenomena can be well interpreted within the framework of a precessing jet model. In this paper, we study the emission polarization of precessing jets in the three kinds of phenomena. The polarization angle also shows a gradual change for the synchrotron emission in both the random and toroidal magnetic field configurations with the precessing jet, while it can only change abruptly by $90^\circ$ for the non-precessing top-hat jet. Polarization properties are periodic due to the assumptions made in our model. The polarization observations are crucial to confirm the precession nature of jets in GRBs, TDEs and AGNs., Comment: 21 pages, 6 figures, accepted by ApJ

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

SN~2007D is a nearby (redshift $z = 0.023146$), luminous Type Ic supernova (SN) having a narrow light curve (LC) and high peak luminosity. Previous research based on the assumption that it was powered by the $^{56}$Ni cascade decay suggested that the inferred $^{56}$Ni mass and the ejecta mass are $\sim 1.5$M$_{\odot}$ and $\sim 3.5$M$_{\odot}$, respectively. In this paper, we employ some multiband LC models to model the $R$-band LC and the color ($V-R$) evolution of SN~2007D to investigate the possible energy sources powering them. We find that the pure $^{56}$Ni model is disfavored; the multiband LCs of SN~2007D can be reproduced by a magnetar whose initial rotational period $P_{0}$ and magnetic field strength $B_p$ are $7.28_{-0.21}^{+0.21}$ (or $9.00_{-0.42}^{+0.32}$) ms and $3.10_{-0.35}^{+0.36}\times 10^{14}$ (or $2.81_{-0.44}^{+0.43}\times 10^{14}$) G, respectively. By comparing the spectrum of SN~2007D with that of some superluminous SNe (SLSNe), we find that it might be a luminous SN like several luminous ``gap-filler" optical transients that bridge ordinary and SLSNe, rather than a genuine SLSN., Comment: 11 pages, 5 figures, 1 table, accepted for publication in ApJ

Very recently \citet{XueYQ2019} reported an important detection of the X-ray transient, CDF-S XT2, whose light curve is analogous to X-ray plateau features of gamma-ray burst afterglows. They suggested that this transient is powered by a remnant stable magnetar from a binary neutron star merger since several pieces of evidence (host galaxy, location, and event rate) all point toward such an assumption. In this paper, we revisit this scenario and confirm that this X-ray emission can be well explained by the internal gradual magnetic dissipation process in an ultra-relativistic wind of the newborn magnetar. We show that both the light curve and spectral evolution of CDF-S XT2 can be well fitted by such a model. Furthermore, we can probe some key properties of the central magnetar, such as its initial spin period, surface magnetic field strength and wind saturation Lorentz factor., Comment: 5 pages, 3 figures, ApJL accepted

The formation of neutron stars (NSs), both from collapses of massive stars and mergers of compact objects, can be usually indicated by bright transients emitted from explosively-ejected material. In particular, if the newborn NSs can rotate at a millisecond period and have a sufficiently high magnetic field, then the spin-down of the NSs would provide a remarkable amount of energy to the emitting material. As a result, super-luminous supernovae could be produced in the massive stellar collapse cases, while some unusual fast evolving and luminous optical transients could arise from the cases of NS mergers and accretion-induced collapses of white dwarfs. In all cases, if the dipolar magnetic fields of the newborn NSs can be amplified to be as high as $10^{15}$ G, a relativistic jet could be launched and then a gamma-ray burst can be produced as the jet successfully breaks out from the surrounding nearly-isotropic ejected material., Comment: 10 pages, 9 pictures, to appear in the AIP Proceedings of the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, China

Plateaus are common in X-ray afterglows of gamma-ray bursts. Among a few scenarios for the origin of them, the leading one is that there exists a magnetar inside and persistently injects its spin-down energy into an afterglow. In previous studies, the radiation efficiency of this process is assumed to be a constant $\gtrsim0.1$, which is quite simple and strong. In this work we obtain the efficiency from a physical point of view and find that this efficiency strongly depends on the injected luminosity. One implication of this result is that those X-ray afterglow light curves which show steeper temporal decay than $t^{-2}$ after the plateau phase can be naturally understood now. Also, the braking indexes deduced from afterglow fitting are found to be larger than those in previous studies, which are more reasonable for newborn magnetars., Comment: 10 pages, 7 figures, accepted by ApJ

GRB 170817A, detected by Fermi-GBM 1.7\,s after the merger of a neutron star (NS) binary, provides the first direct evidence for a link between such a merger and a short-duration gamma-ray burst. The X-ray observations after GRB 170817A indicate a possible X-ray flare with a peak luminosity $L_{\rm peak} \sim 2\times 10^{39}\,{\rm erg\,s}^{-1}$ near day 156. Here we show that this X-ray flare may be understood based on a slim disc around a compact object. On the one hand, there exists the maximal accretion rate $\dot M_{\rm max}$ for the slim disc, above which an optically thick outflow is significant and radiation from the disc is obscured. Based on the energy balance analysis, we find that $\dot M_{\rm max}$ is in the range of $\sim 4\dot M_{\rm Edd}$ and $\sim 21\dot M_{\rm Edd}$ when the angular velocity of the slim disc is between $\rm (1/5)^{1/2}\Omega_K$ and $\rm \Omega_K$ (where $\dot M_{\rm Edd}$ is the Eddington accretion rate and $\Omega_K$ is the Keplerian angular velocity). With $\dot M_{\rm max}$, the slim disc can provide a luminosity $\sim L_{\rm peak}$ for a compact object of $2.5 M_{\sun}$. On the other hand, if the merger of two NSs forms a typical neutrino-dominated accretion disc whose accretion rate $\dot M$ follows a power-law decline with an index $-1.8$ , then the system must pass through the outflow regime and enter the slim disc in $\sim 11-355$ days. These results imply that a post-merger slim accretion disc could account for the observed late-time $L_{\rm peak}$., Comment: 5 pages, 2 figures,accepted for publication in MNRAS

Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard China's Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst., Comment: 34 pages, 15 figures (16 pages and 3 figures without supplementary information). Accepted for publication in Nature Astronomy, http://dx.doi.org/10.1038/s41550-018-0664-0

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., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

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

We investigate emission signatures of binary compact star gravitational wave sources consisting of strongly magnetized neutron stars (NSs) and/or white dwarfs (WDs) in their late-time inspiral phase. Because of electromagnetic interactions between the magnetospheres of the two compact stars, a substantial amount of energy will be extracted, and the resultant power is expected to be $\sim 10^{38} - 10^{44}$ erg/s in the last few seconds before the two stars merge, when the binary system contains a NS with a surface magnetic field $10^{12}$ G. The induced electric field in the process can accelerate charged particles up to the EeV energy range. Synchrotron radiation is emitted from energetic electrons, with radiative energies reaching the GeV energy for binary NSs and the MeV energy for NS - WD or double WD binaries. In addition, a blackbody component is also presented and it peaks at several to hundreds keV for binary NSs and at several keV for NS - WD or double WD binaries. The strong angular dependence of the synchrotron radiation and the isotropic nature of the blackbody radiation lead to distinguishable modulation patterns between the two emission components. If coherent curvature radiation is presented, fast radio bursts could be produced. These components provide unique simultaneous electromagnetic signatures as precursors of gravitational wave events associated with magnetized compact star mergers and short gamma ray bursts (e.g., GRB 100717)., Comment: 16 pages, 8 figures, 1 table. Minor corrections to match the version on ApJ

In this paper, we revisit the scenario that an internal gradual magnetic dissipation takes place within the wind from a newborn millisecond magnetar can be responsible for gamma-ray burst production. We show that a combination of two emission components in this model, i.e., the photospheric emission from the wind and the synchrotron radiation within the magnetic reconnection region, can give a reasonable fit to the observed spectrum of the prompt emission phase of GRB 160804A. We obtain the physical parameters through a Monte Carlo procedure and deduce the initial spin period and magnetic field of the central magnetar. Furthermore, the independent afterglow fitting analysis gives a consistent result, adding great credibility to this scenario. In addition, we predict a subclass of GRBs called bursts from such a Magnetar wind Internal Gradual MAgnetic Dissipation (abbreviated as "MIGMAD bursts") that have several distinctive properties., Comment: Accepted by ApJ, 9 pages, 7 figures

Recent wide-field surveys discovered new types of peculiar optical transients that showed diverse behaviors of the evolution of photospheric properties. We develop a general theory of homologous explosions with constant opacity, paying special attention on the evolution of the photospheric radius $R_{\rm ph}$. We find that regardless of the density distribution profile, $R_{\rm ph}$ always increases early on and decreases at late times. This result does not depend on the radiation and cooling processes inside the ejecta.The general rising/falling behavior of $R_{\rm ph}$ can be used to quickly diagnose whether the source originates from a supernova-like explosion. The shape of the $R_{\rm ph}$ evolution curve depends on the density profile, so the observations may be used to directly diagnose the density profile as well as the temperature profile of the ejecta. All the well-monitored supernovae show such a $R_{\rm ph}$ rising/falling behavior, which is consistent with our theory. The recently discovered peculiar transient AT2018cow showed a continuous decay of $R_{\rm ph}$, which is disfavored to be of a supernova-like explosion origin. Our result therefore supports the interpretation of this transient as a tidal disruption event., Comment: 8 pages, 3 figures, ApJL Accepted

The gravitational wave event GW170817 associated with the short gamma-ray burst (GRB) 170817A confirms that binary neutron star (BNS) mergers are one of the origins of short GRBs. The associated kilonova emission, radioactively powered by nucleosynthesized heavy elements, was also detected. Black hole-neutron star (BH-NS) mergers have been argued to be another promising origin candidate of short GRBs and kilonovae. Numerical simulations show that the ejecta in BH-NS mergers is geometrically much more anisotropic than the BNS merger case. In this paper, we investigate observational signatures of kilonova emission from the anisotropic ejecta in BH-NS mergers. We find that a bump appears on the bolometric luminosity light curve due to the inhomogeneous mass distribution in the latitudinal direction. The decay slope of the single-band light curve becomes flatter and the spectrum also deviates from a single-temperature blackbody radiation spectrum due to the gradient in the velocity distribution of the ejecta. Future detection or non-detection of such signatures would be useful to test the mass ejection geometry in BH-NS mergers., Comment: Accepted by ApJ, 7 pages, 7 figures

A neutrino outburst between September 2014 and March 2015 was discovered from the blazar TXS 0506+056 by an investigation of $9.5$ years of IceCube data, while the blazar was in a quiescent state during the outburst with a gamma-ray flux only about one-fifth of the neutrino flux. In this work, we give a possible interpretation of the abnormal feature by proposing that the neutrino outburst originates from the interaction between a relativistic jet and a dense gas cloud formed via the tidally disrupted envelope of a red giant being blown apart by the impact of the jet. Gamma-ray photons and electron/positron pairs that are produced through the hadronuclear interactions correspondingly will induce electromagnetic cascades and then make the cloud ionized and thermalized. The EM radiation from jet-cloud/star interaction is mainly contributed by the relatively low-energy relativistic protons which propagate in the diffusion regime inside the cloud due to magnetic deflections, whereas the observed high-energy neutrinos ($\gtrsim 100\,\rm TeV$) are produced by the relatively high-energy protons which can keep beamed owing to the weak magnetic deflections, inducing a much higher flux of neutrinos than electromagnetic radiation. The observed low-energy electromagnetic radiations during the neutrino outburst period are almost as same as that in the quiescent state of the source, so it may mainly arise as same as the general quiescent state. As a result, due to the intrusion of a dense cloud, the neutrino outburst can be expected, and in the meantime, the accompanying electromagnetic radiations from hadronic processes will not cause any enhancement in the blazar's electromagnetic flux., Comment: 15 pages, 3 figures; Accepted version

The detection of the high-energy ($\sim290$ TeV) neutrino coincident with the flaring blazar TXS 0506+056, the first and only $3\sigma$ 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., Comment: 5 pages, 1 table, no figures. Accepted by Journal of High Energy Astrophysics

Multi-band observations of the fast radio burst (FRB) 131104 show that this burst may be associated with a gamma-ray transient entitled Swift J0644.5-5111. Follow-up observations for potential X-ray and radio counterparts of FRB 131104/Swift J0644.5-5111 got null results and provided the upper limits of the emission flux at 5.5 GHz, 7.5 GHz, $U$-band, and X-ray band. By assuming this association and using these upper limits, environmental properties (the fraction of energyin a magnetic field $\varepsilon_{\rm B}$ and the number density $n$) of the progenitor system of FRB 131104/Swift J0644.5-5111 were constrained in the context of the standard afterglow model that neglects the non-relativistic effect and jet effect by several groups. In this paper, we adopt a unified afterglow model that takes into account the non-relativistic effect and jet effect and use the upper limits of four bands (5.5 GHz, 7.5 GHz, $U$-band, and X-ray) to obtain more stringent constraints on the parameter space spanned by $\varepsilon_{\rm B}$ and $n$. We thus suggest that FRB 131104/Swift J0644.5-5111 might originate from a black hole-neutron star merger event. Moreover, we calculate multi-band emissions from a kilonova powered by the radioactivity of $r$-process elements synthesized in the ejected neutron-rich material and find that the $U$-band emission from the putative kilonova is significantly lower than the upper limit of the observations., Comment: 9 pages, 2 tables, 6 figures, accepted for publication in ApJ

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 cut-off due to the absorption by magnetic fields, which is much sharper than the exponential one. Below the cut-off, 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_{\perp}, 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., Comment: 7 pages, 7 figures, minor changes. Version to appear in PRD

Gamma-ray bursts (GRBs) have been widely argued to originate from binary compact object mergers or core collapses of massive stars. Jets from these systems may have two components, an inner, narrow sub-jet and an outer, wider sub-jet. Such a jet subsequently interacts with its ambient gas, leading to a reverse shock (RS) and a forward shock (FS). The magnetic field in the narrow sub-jet is very likely to be mixed by an ordered component and a random component during the afterglow phase. In this paper, we calculate light curves and polarization evolution of optical afterglows with this mixed magnetic field in the RS region of the narrow sub-jet in a two-component jet model. The resultant light curve has two peaks: an early peak arises from the narrow sub-jet and a late-time rebrightening is due to the wider sub-jet. We find the polarization degree (PD) evolution under such a mixed magnetic field confined in the shock plane is very similar to that under the purely ordered magnetic field condition. The two-dimensional `mixed' magnetic fields confined in the shock plane are essentially the ordered magnetic fields only with different configurations. The position angle (PA) of the two-component jet can change gradually or abruptly by $90^\circ$. In particular, an abrupt $90^\circ$ change of the PA occurs when the PD changes from its decline phase to rise phase., Comment: 21 pages, 6 figures, accepted by ApJ

Recent follow-up observations of the binary neutron star (NS) merging event GW170817/SGRB 170817A reveal that its X-ray/optical/radio emissions are brightening continuously up to $\sim 100$ days post-merger. This late-time brightening is unexpected from the kilonova model or the off-axis top-hat jet model for gamma-ray burst afterglows. In this paper, by assuming that the merger remnant is a long-lived NS, we propose that the interaction between an electron-positron-pair ($e^+e^-$) wind from the central NS and the jet could produce a long-lived reverse shock, from which a new emission component would rise and can interpret current observations well. The magnetic-field-induced ellipticity of the NS is taken to be $4 \times 10^{-5}$ in our modeling, so that the braking of the NS is mainly through the gravitational wave (GW) radiation rather than the magnetic dipole radiation, and the emission luminosity at early times would not exceed the observational limits. In our scenario, since the peak time of the brightening is roughly equal to the spin-down time scale of the NS, the accurate peak time may help constrain the ellipticity of the remnant NS. We suggest that radio polarization observations of the brightening would help to distinguish our scenario from other scenarios. Future observations on a large sample of short gamma-ray burst afterglows or detections of GW signals from merger remnants would test our scenario., Comment: 7 pages, 1 figure, accepted by ApJL, submitted on January 25, 2018

The high-energy (> 100MeV) emission observed by Fermi-LAT during the prompt phase of some luminous gamma-ray bursts (GRBs) could arise from the cascade induced by interactions between accelerated protons and the radiation field of GRBs. The photomeson process, which is usually suggested to operate in such a hadronic explanation, requires a rather high proton energy (> 10^17eV) for an efficient interaction. However, whether GRBs can accelerate protons to such a high energy is far from guaranteed, although they have been suggested as the candidate source for ultrahigh-energy cosmic rays. In this work, we revisit the hadronic model for the prompt high-energy emission of GRBs with a smaller maximum proton energy than the usually adopted value estimated from the Bohm condition. In this case, the Bethe-Heitler pair production process becomes comparably important or even dominates over the photomeson process. We show that with a relatively low maximum proton energy with a Lorentz factor of 10^5 in the comoving frame, the cascade emission can still reproduce various types of high-energy spectrum of GRBs. For most GRBs without high-energy emission detected, the maximum proton energy could be even lower and relax the constraints on the parameters of GRB jet resulting from the fact of non-detection of GRB neutrinos by IceCube., Comment: 36 pages, 13 figures, accepted for publication in ApJ

In this paper, we investigate two hydrogen-poor superluminous supernovae (SLSNe) iPTF15esb and iPTF13dcc whose light curves (LCs) show significant deviation from the smooth rise and fall. The LC of iPTF15esb exhibits two peaks and a post-peak plateau, and furthermore the late-time spectrum of iPTF15esb shows a strong, broad H$\alpha$ emission line. The early-time LC of iPTF13dcc shows a long duration bump followed by the second peak. Here we propose an ejecta-circumstellar medium (CSM) interaction model involving multiple shells/winds and use it to explain the LCs of iPTF15esb and iPTF13dcc. We find that the theoretical LCs reproduced by this model can well match the observations of iPTF15esb and iPTF13dcc. Based on our results, we infer that the progenitors have undergone multiple violent mass-loss processes before the SN explosion. In addition, we find that the variation trend of our inferred densities of the shells is consistent with that predicted by the stellar mass-loss history before an SN explosion. Further investigations for other bumpy SLSNe/SNe would shed light on their nature and provide a probe for the mass-loss history of their progenitors., Comment: 20 pages, 4 figures, 2 tables, accepted for publication in ApJ. arXiv admin note: substantial text overlap with arXiv:1706.01783

The Intermediate Palomar Transient Factory reported the discovery of an unusual type II-P supernova iPTF14hls. Instead of a ~100-day plateau as observed for ordinary type II-P supernovae, the light curve of iPTF14hls has at least five distinct peaks, followed by a steep decline at ~1000 days since discovery. Until 500 days since discovery, the effective temperature of iPTF14hls is roughly constant at 5000-6000K . In this paper we propose that iPTF14hls is likely powered by intermittent fallback accretion. It is found that the light curve of iPTF14hls can be well fit by the usual t^{-5/3} accretion law until ~1000 days post discovery when the light curve transitions to a steep decline. To account for this steep decline, we suggest a power-law density profile for the late accreted material, rather than the constant profile as appropriated for the t^{-5/3} accretion law. Detailed modeling indicates that the total fallback mass is ~0.2M_{sun}, with an ejecta mass M_{ej}~21M_{sun}. We find the third peak of the light curve cannot be well fit by the fallback model, indicating that there could be some extra rapid energy injection. We suggest that this extra energy injection may be a result of a magnetic outburst if the central object is a neutron star. These results indicate that the progenitor of iPTF14hls could be a massive red supergiant., Comment: 8 pages, 2 figure, 2 tables. The published version

Limited by the sensitivities of the current gravitational wave (GW) detectors, the central remnant of the binary neutron star (NS) merger associated with GW170817 remains an open question. Considering the relatively large total mass, it is generally proposed that the merger of GW170817 would lead to a shortly lived hypermassive NS or directly produce a black hole (BH). There is no clear evidence to support or rule out a long-lived NS as the merger remnant. Here we utilize the GW and electromagnetic (EM) signals to comprehensively investigate the parameter space that allows a long-lived NS to survive as the merger remnant of GW170817. We find that for some stiff equations of state, the merger of GW170817 could, in principle, lead to a massive NS, which has a millisecond spin period. The post-merger GW signal could hardly constrain the ellipticity of the NS. If the ellipticity reaches $10^{-3}$, in order to be compatible with the multi-band EM observations, the dipole magnetic field of the NS ($B_p$) is constrained to the magnetar level of $\sim10^{14}$ G. If the ellipticity is smaller than $10^{-4}$, $B_p$ is constrained to the level of $\sim10^{10}-10^{12}\,$G. These conclusions weakly depend on the adoption of equations of state., Comment: 8 pages, 1 figures

A subset of short gamma-ray bursts (sGRBs) have been found to be characterized by near-infrared/optical bumps at $\sim 1$ days, some of which exhibit almost concurrent X-ray flares. Although the near-infrared/optical bumps may be a signature of kilonovae, the X-ray flares are not consistent with kilonovae. It is widely believed that sGRBs are produced by the mergers of double compact objects, during which sub-relativistic ejecta are launched. In this paper we propose that the above optical/X-ray features are indicative of the formation of long-lived magnetars following the mergers of double neutron stars. Observations and theoretical works imply that the spin-down power of the magnetars is injected into the ejecta as ultra-relativistic electron-positron pairs, i.e. pulsar wind nebulae (PWNe). Here we suggest such a PWN model and find that the optical bump and X-ray flare observed in GRB 050724 can be well understood in this PWN model. We show that the optical bump and X-ray flare may have different origins. Our results strengthen the evidence for the formation of magnetars in double neutron star mergers and justify the validity of the PWN model., Comment: 8 pages, 5 figures, 1 table, published in The Astrophysical Journal

The first gravitational-wave event from the merger of a binary neutron star system (GW170817) was detected recently. The associated short gamma-ray burst (GRB 170817A) has a low isotropic luminosity ($\sim 10^{47}$ erg s$ ^{-1}$) and a peak energy $E_{p}\sim 145$ keV during the initial main emission between -0.3 and 0.4 s. The origin of this short GRB is still under debate, but a plausible interpretation is that it is due to the off-axis emission from a structured jet. We consider two possibilities. First, since the best-fit spectral model for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy photon index ($\alpha =-0.62_{-0.54}^{+0.49} $), we consider an off-axis photosphere model. We develop a theory of photosphere emission in a structured jet and find that such a model can reproduce a low-energy photon index that is softer than a blackbody through enhancing high-latitude emission. The model can naturally account for the observed spectrum. The best-fit Lorentz factor along the line of sight is $\sim 20$, which demands that there is a significant delay between the merger and jet launching. Alternatively, we consider that the emission is produced via synchrotron radiation in an optically thin region in an expanding jet with decreasing magnetic fields. This model does not require a delay of jet launching but demands a larger bulk Lorentz factor along the line of sight. We perform Markov Chain Monte Carlo fitting to the data within the framework of both models and obtain good fitting results in both cases.

The central engine of gamma-ray bursts (GRBs) is poorly constrained. There exist two main candidates: a fast-rotating black hole and a rapidly spinning magnetar. Furthermore, the X-ray plateaus are widely accepted by the energy injection into the external shock. In this paper, we systematically analyze the \emph{Swift}/XRT light curves of 101 GRBs having plateau phases and known redshifts (before May 2017). Since a maximum energy budget ($\sim2\times10^{52}$ erg) exists for magnetars but not for a black hole, this provides a good clue to identify the type of GRB central engine. We calculate the isotropic kinetic energy $E_{\rm K,iso}$ and the isotropic X-ray energy release $E_{\rm X,iso}$ for individual GRB. We identify three categories based on how likely a black hole harbor at central engine: 'Gold' (9 out of 101, both $E_{\rm X,iso}$ and $E_{\rm K,iso}$ exceed the energy budget), 'Silver' (69 out of 101, $E_{\rm X,iso}$ less than the limit but $E_{\rm K,iso}$ is greater than the limit), and 'Bronze' (23 out of 101, the energies are not above the limit). We then derive and test the black hole parameters with the Blandford Znajek mechanism, and find that the observations of the black hole candidate ('Gold'+'Silver') samples are consistent with the expectations of the black hole model. Furthermore, we also test the magnetar candidate ('Bronze') sample with the magnetar model, and find that the magnetar surface magnetic led ($B_{p}$) and initial spin period ($P_{0}$) are consistent with the expectations of the magnetar model. Our analysis indicates that, if the magnetar wind is isotropic, a magnetar central engine is possible for 20\% of the analyzed GRBs. For most GRBs a black hole is most likely operating., Comment: 61 pages, 134 Figures, 5 Tables, accepted for publication in ApJS

The successful joint observation of the gravitational wave event GW170817 and its multi-wavelength electromagnetic counterparts first enables human to witness a definite merger event of two neutron stars (NSs). This historical event confirms the origin of short-duration gamma-ray bursts (GRBs), and in particular, identifies the theoretically-predicted kilonova phenomenon that is powered by radioactive decays of $r$-process heavy elements. However, whether a long-lived remnant NS could be formed during this merger event remains unknown, although such a central engine has been suggested by afterglow observations of some short-duration GRBs. By invoking this long-lived remnant NS, we here propose a model of hybrid energy sources for the kilonova AT2017gfo associated with GW 170817. While the early emission of AT2017gfo is still powered radioactively as usually suggested, its late emission is primarily caused by delayed energy injection from the remnant NS. In our model, only one single opacity is required and an intermediate value of $\kappa\simeq0.97\,\rm cm^2g^{-1}$ is revealed, which could be naturally provided by lanthanide-rich ejecta that is deeply ionized by the emission from a wind of the NS. These self-consistent results indicate that a long-lived remnant NS, which must own a very stiff equation of state, had been formed during the merger event of GW170817. This provides a very stringent constraint on the strong interaction in nuclear-quark matter. It is further implied that such GW events could provide a probe of the early spin and magnetic evolutions of NSs, e.g., the burying of surface magnetic fields., Comment: 10 pages, 4 figures. Following the referee's suggestions, the MCMC fitting method is described and the discussions on the EOS and magnetic field burying of the remnant NS are expanded. Accepted for publication in ApJ

The coincident detection of a gravitational-wave (GW) event GW170817 with electromagnetic (EM) signals (e.g., a short gamma-ray burst SGRB 170817A or a macronova) from a binary neutron star merger within the nearby galaxy NGC 4933 provides a new, multimessenger test of the weak equivalence principle (WEP), extending the WEP test with GWs and photons. Assuming that the arrival time delay between the GW signals from GW170817 and the photons from SGRB 170817A or the macronova is mainly attributed to the gravitational potential of the Milky Way, we demonstrate that the strict upper limits on the deviation from the WEP are $\Delta \gamma<1.4\times10^{-3}$ for GW170817/macronova and $\Delta \gamma <5.9\times10^{-8}$ for GW170817/SGRB 170817A. A much more severe constraint on the WEP accuracy can be achieved ($\sim0.9\times10^{-10}$) for GW170817/SGRB 170817A when we consider the gravitational potential of the Virgo Cluster, rather than the Milky Way's gravity. This provides the tightest limit to date on the WEP through the relative differential variations of the $\gamma$ parameter for two different species of particles. Compared with other multimessenger (photons and neutrinos) results, our limit is 7 orders of magnitude tighter than that placed by the neutrinos and photons from supernova 1987A, and is almost as good as or is an improvement of 6 orders of magnitude over the limits obtained by the low-significance neutrinos correlated with GRBs and a blazar flare., Comment: 10 pages, no figures. Accepted by JCAP on Nov 8, 2017

Very recently, the gravitational-wave (GW) event GW170817 was discovered to be associated with the short gamma-ray burst (GRB) 170817A. Multi-wavelength follow-up observations were carried out, and X-ray, optical, and radio counterparts to GW170817 were detected. The observations undoubtedly indicate that GRB 170817A originates from a binary neutron star merger. However, the GRB falls into the low-luminosity class that could have a higher statistical occurrence rate and detection probability than the normal (high-luminosity) class. This implies the possibility that GRB 170817A is intrinsically powerful, but we are off-axis and only observe its side emission. In this Letter, we provide a timely modeling of the multi-wavelength afterglow emission from this GRB and the associated kilonova signal from the merger ejecta, under the assumption of a structured jet, a two-component jet, and an intrinsically less-energetic quasi-isotropic fireball, respectively. Comparing the afterglow properties with the multi-wavelength follow-up observations, we can distinguish between these three models. Furthermore, a few model parameters (e.g., the ejecta mass and velocity) can be constrained., Comment: 13 pages, 6 figures, 2 tables, accepted for publication in ApJ Letters. arXiv admin note: substantial text overlap with arXiv:1710.00275

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