The standard external-shock model, assuming a homogeneous turbulent downstream, has been widely used to decipher the afterglows of gamma-ray bursts. However, such assumption is invalid when the shock encounters a density jump. In this paper, we propose a self-consistent scenario to model the external forward shock emission, involving the advection and decaying behaviors of the shock-generated magnetic fields (sgMFs) in the downstream as found in particle-in-cell simulations. In an interstellar medium, our model is almost returned to the standard model but with & varepsilon;B proportional to(1+tdyn/tB)alpha t . Here, & varepsilon;B describes the sgMFs' fraction of shock energy in the standard model, tdyn is the shock dynamic time, and tB together with alpha t depicts the sgMFs' decaying behavior in our model. The situation is the same for the wind medium but with weak deviation in the early phase. When the shock encounters the density rise/dip, a shallower/deeper decay appears in the light curve. These behaviors are obvious for high-frequency emission or at the phase after the jet break. The GeV emission in the afterglow can serve to probe the circumburst density jump. We apply our model to decipher the later afterglows of the giant flare from SGR 1806-20. It is shown that a fireball propagating into a magnetar bow shock environment can well reproduce the observed peculiar steep decay in the radio light curve at similar to 10 days from the burst.

The jet compositions of gamma-ray bursts (GRBs) are very important to understand the energy dissipation and radiation mechanisms, but it remains an open question in GRB physics. In this paper, we present a systematic analysis to search for 88 bright GRBs that include a total of 129 pulses observed by Fermi/GBM with redshift measured, and extract the spectra of each pulse with Band function (Band), cutoff power-law (CPL), blackbody (BB), non-dissipative photospheric (NDP), Band + BB, as well as CPL + BB. We find that 80 pulses, 35 pulses, and 14 pulses present purely non-thermal, hybrid, and thermal spectra, respectively. By focusing on those 80 pulses with purely non-thermal spectra, one can estimate the lower limits of magnetization factor ( sigma) via suppressing the pseudo-thermal component. It is found that 30 pulses in 21 GRBs are the lower limit of sigma > 5 at the photosphere by adopting R-0 = 10(10) cm. It suggests that at least the outflow of those GRB jets with high sigma is dominated by Poynting-flux. On the other hand, we also perform the light curve fitting with a fast-rise-exponential-decay (FRED) model for 15 bright GRBs with a high magnetization factor in our sample, and find that a correlation between pulse width ( w ) and energy of 13 GRBs really exists in their energy-resolved light curves. It is also a piece of independent evidence for those GRBs with a high value sigma to support the origin of the Poynting flux outflow.

Studying the periodic flux-variation behaviour of blazars is vital for probing supermassive black hole binaries and the kinematics of relativistic jets. In this work, we report the detection of multi-band possible periodic variations of the blazar PKS J2134-0153, including infrared (1.6(+/- 0.4)x10(3) day) and optical (1.8(+/- 1)x10(3 )day) wavelengths. The periods in the infrared and optical bands are statistically consistent with the period in the radio band (P-Radio=1760 +/- 33 days, obtained from our previous work). Moreover, flux variations in different bands are correlated with evident inter-band time delays, and the time lags of infrared and optical emission with respect to radio emission are (3.3 +/- 2.3)x10(2) days and (3.0 +/- 2.3)x10(2) days, respectively. The cross-correlations indicate a common origin of radio, infrared, and optical emission. The relative positions between emission regions of infrared and optical emission and radio emission are estimated according to the time lags, i.e. 0.37 +/- 0.26 and 0.33 +/- 0.26 pc. The relative distances seem to be quantitatively consistent with the theoretical prediction.

The gamma-ray light curve of long-duration GRB 220711B, is characterized by a multipeaked structure with a duration lasting '105 s. More interestingly, the X-ray afterglow light curve is composed of a plateau emission smoothly connected with a similar to t(-2) segment overlapping some flares followed by an extremely steep decay. By analysing the light curves of both prompt emission and X-ray afterglow, no high-confidence-level quasi-periodic oscillation (QPO) signals are found in the light curves of the prompt emission (e.g. Burst Alert Telescope and Gamma-ray Burst Monitor), but it is found that a QPO signal at '50 s above 6cr confidence level indeed exist in the X-ray afterglow. Here, we propose that a supra-massive magnetar as the central engine of GRB 220711B with precession motion is a good interpretation of the features of the X-ray emission. The initial plateau emission and followed decay segment, as well as the extremely steep-decay segment, are consistent with the physical process of supra-massive magnetar spin-down and then collapse into black hole. Moreover, the QPO signal in the X-ray emission can be explained as an effect of the precession motion of the magnetar. If this is the case, one can derive various magnetar parameters such as the initial period ( P-0 ) and surface magnetic field strength ( B-p ) within a pseudo-redshift range of [1.08, 4.27]. By considering beaming corrections with jet opening angle 5 degrees, we find that P-0 and B-p lie within the range of [1.87, 6.25] ms and [1 . 47 x 10(16) , 3 . 09 x 10(16) ] G, respectively. The parameter of Bp is slightly larger than that of other typical long-duration GRBs, but P-0 fall in a reasonable range.

We report a high-redshift (z=1.404) tidal disruption event (TDE) candidate in SDSS J000118.70+003314.0 (SDSS J0001), which is a quasar with apparent broad Mg II emission line. The long-term variability in its nine-year photometric ugriz-band light curves, obtained from the SDSS Stripe82 and the PHOTOOBJALL data bases, can be described by the conventional TDE model. Our results suggest that the TDE is a main-sequence star with mass of 1.905(-0.009)(+0.023) M-circle dot tidally disrupted by a black hole (BH) with mass 6.5(-2.6)(+3.5)x 10(7 )M(circle dot). The BH mass is about 7.5 times smaller than the virial BH mass derived from the broad Mg ii emission line, which can be explained by non-virial dynamic properties of broad emission lines from TDEs debris. Furthermore, we examine the probability that the event results from intrinsic variability of quasars, which is about 0.009 per cent, through applications of the DRW/CAR process. Alternative explanations for the event are also discussed, such as the scenarios of dust obscurations, microlensing and accretion. Our results provide clues to support that TDEs could be detectable in broad line quasars as well as in quiescent galaxies, and to indicate the variability of some active galactic nuclei may be partly attributed to central TDEs.

Rapidly rotating newborn magnetars, which originate from binary neutron star (NS) mergers and serve as the central engines of short gamma-ray bursts (GRBs), may leave some imprints on their prompt gamma-ray lightcurves even though they are far from their radiating fireballs. A high-frequency quasi-periodic oscillation (QPO) would be a unique feature for the magnetar central engine, especially a hypermassive magnetar. By conducting a systematic analysis of the prompt gamma-ray lightcurves from 605 short GRBs observed by Fermi/GBM, we have identified such QPO signals in three GRBs (e.g. GRB 120323A, GRB 181222B, and GRB 190606A). The QPOs that peaked at $1258^{+6}_{-6}$ Hz for GRB 120323A, $623^{+4}_{-4}$ Hz for GRB 181222B, and $1410^{+4}_{-5}$ Hz for GRB 190606A are all with a confidence level above 5.2 σ. The high-frequency QPO signals of those three short GRBs may be caused by a hypermassive magnetar acting as the central engine in a binary neutron star merger of binary neutron star.

Theoretically, a supra-massive neutron star or magnetar may be formed after the merger of binary neutron stars. GRB 210323A is a short-duration gamma-ray burst (GRB) with a duration of lasting similar to 1 s. The light curve of the prompt emission of GRB 210323A shows a signal-peaked structure and a cutoff power-law model can adequately fit the spectra with E p = 1826 +/- 747. More interestingly, it has an extremely long-lasting plateau emission in the X-ray afterglow with a duration of similar to 10(4) s, and then follows a rapid decay with a decay slope similar to 3.2. This temporal feature is challenging by invoking the external shock mode. In this paper, we suggest that the observed long-lasting X-ray plateau emission is caused by the energy injection of dipole radiation from supra-massive magnetar, and the abrupt decay following the long-lasting X-ray plateau emission is explained by supra-massive magnetar collapsing into a black hole. It is the short GRB (SGRB) with the longest X-ray internal plateau emission powered by a supra-massive neutron star. If this is the case, one can estimate the physical parameters of a supra-massive magnetar, and compare with other SGRBs. We also discuss the possible gravitational-wave emission, which is powered by a supra-massive magnetar and its detectability, and the possible kilonova emission, which is powered by r-process and magnetar spin-down to compare with the observed data.

Short gamma-ray bursts (SGRBs) with extended emission (EE) are composed of initial main emission with a short-hard spike (ME) and followed by a long-lasting EE. Whether the ME and EE originated from the same origin or not, as well as the jet composition, remains an open question. In this paper, we present a systematic analysis to search for 36 GRBs in our sample, which are identified as the category of SGRBs with EE as observed by Fermi/GBM. By extracting time-integrated spectra of ME and EE with CPL or Band models for our sample, we find that 20 out of 36 SGRBs for which α values are exceeding the death line (e.g., -2/3) of synchrotron emission within either ME or EE phases, and suggest that the quasi-thermal component should exist in the prompt emission. Then, we extract the time-resolved spectra of our samples, but only four GRBs are bright enough to extract the time-resolved spectra. We find that both thermal and non-thermal emissions do exist in the prompt emission of those four bright GRBs, and it suggests that a hybrid jet (e.g., matter and Poynting-flux outflow) in GRB should exist. Moreover, strong positive correlations (e.g., F_ tot-Γ and F_ tot-kT) in the time-resolved spectra of ME and EE for those four GRBs are discovered. It indicates that the spectral evolution of both ME and EE seem to share similar behavior, possibly from the same physical origin.

The γ-ray emitting compact symmetric objects(CSOs)PKS 1718-649,NGC 3894,and TXS 0128+554 are lobe-dominated in the radio emission.In order to investigate their γ-ray radiation properties,we analyze the～14yr Fermi/LAT observation data of the three CSOs.They all show the low luminosity(1041-1043 erg s-1)and no significant variability in the γ-ray band.Their γ-ray average spectra can be well fitted by a power-law function.These properties of γ-rays are clearly different from the γ-ray emitting CSOs CTD 135 and PKS 1413+135,for which the γ-rays are produced by a restarted aligned jet.In the Lγ-Γγ plane,the three CSOs are also located at the region occupied by radio galaxies(RGs)while CTD 135 and PKS 1413+135 display a similar feature to blazars.Together with a similar radio emission property to γ-ray emitting RGs Cen A and Fornax A,we speculate that theγ-rays of the three CSOs stem from their extended mini-lobes.The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model,where their γ-rays are produced by the inverse Compton process of the relativistic electrons in extended regions.By extrapolating the observed Fermi/LAT spectra to the very high energy band,we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in the future.

Recently, the detection of a coherent radio flash associated with short-duration GRB 201006A, occurring 76.6 minutes after the burst, has attracted great attention. However, the physical origin of the coherent radio flash remains under debate. By reanalyzing its data observed by Fermi and Swift, we find that an early radio afterglow as the physical origin of the radio flash can be ruled out, but the coherent radio emission seems to be consistent with the hypothesis of a supramassive magnetar as the central engine collapsing into a black hole. Within this scenario, the derived magnetar surface magnetic field (B p ) and the initial spin period (P 0) fall into a reasonable range but require a preferably low value of eta R = 10-7 or 10-6. Moreover, the calculated low-epsilon value and E gamma,iso-E p correlation of GRB 201006A also supports the progenitor which is from the merger of compact stars. We also discuss the non-detected kilonova emission associated with GRB 201006A, and then compare with its upper limits of optical observations.

Long-duration gamma-ray bursts (GRBs) are thought to be from core collapse of massive stars, and a rapidly spinning magnetar or black hole may be formed as the central engine. The extended emission in the prompt emission, flares, and plateaus in X-ray afterglow, are proposed to be as the signature of central engine re-activity. However, the direct evidence from observations of identifying the central engines remains an open question. In this paper, we systemically search for long-duration GRBs that consist of bumps in X-ray afterglow detected by Swift/XRT and find that the peak time of the X-ray bumps exhibit bimodal distribution (defined as 'early' and 'late' bumps) with division line at t = 7190 s. Although we cannot rule out that such a bimodality arises from selection effects. We proposed that the long-duration GRBs with an early (or late) bumps may be originated from the fall-back accretion onto a new-born magnetar (or black hole). By adopting Monte Carlo Markov Chain (MCMC) method to fit the early (or late) bumps of X-ray afterglow with the fall-back accretion of magnetar (or black hole), it is found that the initial surface magnetic field and period of magnetars for most early bumps are clustered around 5.88 x 10(13) G and 1.04 ms, respectively. Meanwhile, the derived accretion mass of black hole for late bumps is in the range of [4 x 10(-4) , 1.8 x 10(-2)]M-Theta, and the typical fall-back radius is distributed range of [1.04 , 4.23] x 10(11) cm, which is consistent with the typical radius of a Wolf-Rayet star. However, we also find that the fall-back accretion magnetar model is disfavoured by the late bumps, but the fall-back accretion of black hole model cannot be ruled out to interpret the early bumps of X-ray afterglow.

The prompt emission, X-ray plateau, and X-ray flares of gamma-ray bursts (GRBs) are thought to be from internal dissipation, and the magnetar as the central engine with propeller fallback accretion is proposed to interpret the observed phenomena of GRBs. In this paper, by systematically searching for X-ray emission observed by Swift/X-ray Telescope, we find that seven robust GRBs include both X-ray flares and plateau emissions with measured redshift. More interestingly, the X-ray flares/bumps for those seven GRBs are simultaneously observed in the gamma-ray band. By adopting the propeller fallback accretion model to fit the observed data, it is found that the free parameters of two GRBs (140512A and 180329B) can be constrained very well, while in the other five cases, more or less, they are not all sufficiently constrained. On the other hand, this requires the conversion efficiency of the propeller to be two or three times higher than that of the spindown dipole radiation of the magnetar. If this is the case, it is contradictory to the expectation from the propeller model: namely, a dirtier ejecta should be less efficient in producing gamma-ray emissions. Our results hint that at least the magnetar central engine with propeller fallback accretion model cannot interpret very well both the GRB X-ray flares simultaneously observed in the gamma-ray band and the X-ray flares of GRBs with a high Lorentz factor.

Recently, a lack of supernova-associated with long-duration gamma-ray burst (GRB 230307A) at such a low redshift z = 0.065, but associated with a possible kilonova emission, has attracted great attention. Its heavy element nucleosynthesis and the characteristic of soft X-ray emission suggest that the central engine of GRB 230307A is a magnetar that is originated from a binary compact star merger. The calculated lower value of ε ∼ 0.05 suggests that GRB 230307A seems to have an ambiguous progenitor. The lower value of f _eff = 1.23 implies that GRB 230307A is not likely to be from the effect of “tip of iceberg.” We adopt the magnetar central engine model to fit the observed soft X-ray emission with varying efficiency and find that the parameter constraints of the magnetar falls into a reasonable range, i.e., B < 9.4 × 10 ^15 G and P < 2.5 ms for Γ _sat = 10 ^3 , and B < 3.6 × 10 ^15 G and P < 1.05 ms for Γ _sat = 10 ^4 . Whether the progenitor of GBR 230307A is from the mergers of neutron star–white dwarf (NS–WD) or neutron star–neutron star (NS–NS) remains unknown. The difference of GW radiation between NS–NS merger and NS–WD merger may be a probe to distinguish the progenitor of GRB 230307A-like events in the future.

Long-duration GRB 211211A that lacks a supernova emission even down to very stringent limits at such a low redshift z = 0.076 and is associated with kilonova emission, suggests that its physical origin is from a binary compact star merger. By reanalyzing its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that both time-integrated and time-resolved spectra can be fitted well by using a 2SBPL plus blackbody (2SBPL+BB) model in the prompt emission. The bulk Lorentz factors (Γ ph ) of the outflow can be inferred by invoking the observed thermal emission at the photosphere radius within a pure fireball model, and we find out that the temporal evolution of Γ ph seems to be tracking with the light curve. The derived values of Γ ph are also consistent with the Γ ph – L γ ,iso / E γ ,iso correlations that had been found in other bursts. Moreover, we also calculate the magnetization factor σ 0 in the central engine and σ ph at the photosphere radius within the framework of a hybrid jet model, and find that the values of both 1 + σ 0 and 1 + σ ph are larger than 1 for different time slices. It suggests that at least the Poynting-flux component is indeed existent in the outflow. If this is the case, one possible physical interpretation of thermal and nonthermal emissions in GRB 211211A is from the contributions of both ν ν ¯ annihilation and the Blandford–Znajek mechanisms in the relativistic jet when a stellar mass black hole resides in the central engine.

It is proposed that the synchrotron emission from an internal shock with a decaying shock-generated magnetic field can account for the prompt emission of gamma-ray bursts (GRBs). Generally, a jet from the central engine of a GRB is launched with a significant magnetization, and thus there would be a background magnetic field, rather than only the shock-generated magnetic field, in the emission region. In this paper, we study the synchrotron emission of internal shocks with both a decaying shock-generated magnetic field and a nondecaying background magnetic field. It is found that a shoulder with spectral index −1/2 appears in the low-energy regime of the radiation spectrum. The shoulder becomes dominant by increasing the ratio of the background magnetic field energy to the initial value of the shock-generated magnetic field energy f _B . Correspondingly, a radiation spectrum with two bumps or a plateau around the peak of the ν F _ν − ν spectrum may appear. Owing to the decay of the shock-generated magnetic field, the radiation spectral morphology in the high-energy regime is not a power-law function even though a power-law distribution of electrons is injected. We apply our model to GRB 211211A, of which the hard main emission is suggested to originate from the synchrotron emission. Compared with the spectral fitting results with a Band function and the synchrotron emission from the standard straightforward internal shocks, our model presents a perfect fitting to the observations. The fitting results show that f _B is around 0.41–0.99 for the hard main emission of this burst.

The detection of quasi-periodic oscillations(QPOs)in magnetar giant flares(GFs)has brought a new perspective to studies of the mechanism of magnetar bursts.Due to the scarcity of GFs,searching for QPOs in magnetar short bursts is reasonable.Here we report the detection of a narrow QPO at approximately 110 Hz and a wide QPO at approximately 60 Hz in the short magnetar burst SGR 150228213,with a confidence level of 3.35σ.This burst was initially attributed to 4U 0142+61 by Fermi/GBM on location,but we have not detected such QPOs in other bursts from this magnetar.We also found that there was a repeating fast radio burst associated with SGR 150228213 on location.Finally,we discuss the possible origins of SGR 150228213.

Long gamma-ray bursts, which indicate the end-life collapse of very massive stars, are produced by extremely relativistic jets colliding with circumstellar medium. A huge amount of energy is released both in the first few seconds, namely the internal dissipation phase, which powers prompt emissions, and in the subsequent self-similar jet-deceleration phase, which produces afterglows observed in the broadband electromagnetic spectrum. However, prompt optical emissions of gamma-ray bursts have rarely been detected, seriously limiting our understanding of the transition between the two phases. Here we report detection of prompt optical emissions from a gamma-ray burst (that is, GRB 201223A) using a dedicated telescope array with a high temporal resolution and a wide time coverage. The early phase coincident with prompt gamma-ray emissions shows a luminosity in great excess with respect to the extrapolation of gamma-rays, while the later luminosity bump is consistent with onset of the afterglow. The clearly detected transition allows us to differentiate physical processes contributing to early optical emissions and to diagnose the composition of the jet. Rare early optical observations that captured the prompt-to-afterglow emission of GRB 201223A demonstrate that it behaved according to the fireball model of gamma-ray bursts.

We systematically search for quasiperiodic oscillatory (QPO) signals on the month timescale among the 1525 sources given in the Fermi Large Area Telescope Light Curve Repository. We find a transient QPO of 31.3 ± 1.8 days in the gamma-ray band light curve of the TeV blazar S5 0716+714, which has seven cycles (MJD 55918–56137) for the first time by weighted wavelet Z-transform and Lomb–Scargle periodogram methods. Monte Carlo simulations based on the power spectral density and probability distribution function were used to evaluate the confidence level of the QPO, and the result is ∼4.1σ. Seasonal autoregressive integrated moving average modeling of the light curve revealed it is a significant physical QPO. The physical models to explain the sporadic month-timescale QPOs in the blazar were discussed. Our studies indicate that the helical jet model and blob move helically in a curved jet model to properly explain this kind of transient QPO.

In this work, we have assembled the long-term variability data of the blazar 4FGL J0650.7 + 2503 in the gamma-ray and the optical bands, spanning about 11.9 and 8.6 years, respectively. The light curves are then analyzed by using Lomb-Scargle Periodogram, Weighted Wavelet Z-transform, Jurkevich and discrete correlation function techniques, and the results reveal two possible timescales of quasi-periodic oscillation: 500 +/- 37 days for gamma-ray and 330 +/- 20 days for optical. To explore the origin of the gamma-ray, we investigated between the optical and gamma-ray band correlations, and found that the correlation between the two bands is very significant. This correlation can be reasonably explained by the lepton self-synchro-Compton model. Based on the supermassive binary black hole system model, we estimate the primary black hole mass M similar to 8.5 x 10(8) M-circle dot.

We report on a very bright, long-duration gamma-ray burst (GRB), GRB 220426A, observed by Fermi satellite. GRB 220426A with total duration of T_90=6 s is composed with two main pulses and some sub-peaks. The spectral analysis of this burst with Band function reveals that both the time-integrated and the time-resolved spectra are very narrow with high α≳ 0.2 and low β≲ -3.1. It is strong reminiscent of GRB 090902B, a special GRB with identification of the photospheric emission. Then, we perform the spectral analysis of this burst based on a non-dissipated photospheric emission, which can be well modelled as the multicolor-blackbody with a cutoff power-law distribution of the thermal temperature. The spectral fittings reveal that the photospheric emission can well describe the radiation spectrum of this burst. We conclude that this burst would be a second burst in the class of GRB 090902B observed by Fermi satellite. We also discuss the physics of photosphere and the origin of the high-energy component in GRB 220426A .