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Short gamma-ray bursts (GRBs) are explosive transients caused by binary mergers of compact objects containing at least one neutron star. Multiwavelength afterglow observations provide constraints on the physical parameters of the jet, its surrounding medium, and the microphysics of the enhanced magnetic fields and accelerated electrons in the blast wave at the front of the jet. The synchrotron radio emission can be tracked for much longer than in other spectral regimes, and it can pin down the evolution of the spectral peak. We present the results of a systematic observing campaign of eight short GRBs with the MeerKAT radio telescope. Additionally, we present observations of four of these short GRBs using the ATCA radio telescope and two of these short GRBs with the e -MERLIN radio telescope. Using these results we report one possible detection of a short GRB afterglow from GRB 230217A and deep upper limits for the rest of our short GRB observations. We use these observations to place constraints on some of the physical parameters, in particular those related to electron acceleration, the circumburst density, and gamma-ray energy efficiency. We discuss how deeper observations with new and upgraded telescopes should be able to determine if the gamma-ray efficiency differs between long and short GRBs. We also report detections of the likely host galaxies for four of the eight GRBs and upper limits for another GRB, increasing the number of detected host galaxies in the radio with implications for the star formation rate in these galaxies. [ABSTRACT FROM AUTHOR]

The absorption of a gamma-ray burst (GRB) afterglow in the X-ray wavelength range in a dense molecular cloud is investigated. We present the results of our numerical simulations of the propagation of GRB radiation in the cloud for various gas densities, metallicities, and distances from the GRB progenitor star to the cloud. We consider a sample of 45 GRBs with known redshifts in which the isotropic-equivalent gamma-ray energy corresponds to the value adopted in our numerical simulations. For these GRBs we have analyzed the Swift/XRT energy spectra of their afterglows at late times, s. We show that the hydrogen column densities estimated using the absorption model in which the ionization of metal ions is ignored and solar metallicity is used are a factor of 1–3 lower than the actual values if the molecular cloud is near the GRB progenitor star. If the gas cloud is located at a distance pc from the GRB source or the gas metallicity , then the influence of the could ionization structure on the afterglow absorption is minor. [ABSTRACT FROM AUTHOR]

The milestone discovery of GW170817-GRB 170817A-AT 2017gfo has shown that gravitational waves (GWs) could be produced during the merger of a neutron star–neutron star/black hole and that in electromagnetic (EM) waves, a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM properties before the merger phase are still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts ∼0.2 s, and the waiting time between the precursor and the main burst is ∼1 s, comparable to that between GW170817 and GRB 170817A. The spectrum of the precursor could be well fit with a nonthermal cutoff power-law model instead of a blackbody. In particular, a ∼22 Hz quasiperiodic oscillation candidate (∼3 σ) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanied with magnetoelastic or crustal oscillations of a magnetar in a binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before the merger. [ABSTRACT FROM AUTHOR]

The time-integrated polarization degree (PD) in the prompt optical band of a gamma-ray burst (GRB) was predicted to be less than 20%, while the time-resolved one can reach as high as 75% in the photosphere model. Polarizations in the optical band during the GRB prompt phase have not previously been studied in the framework of the magnetic reconnection model. Here, a three-segment power law of the energy spectrum is used to reconstruct the Stokes parameters of the magnetic reconnection model. Multiwavelength light curves and polarization curves from the optical band to MeV gamma rays in the GRB prompt phase are studied. We found that, depending mainly on the jet dynamics, there is a long-lasting high-PD phase in all calculated energy bands for the typical parameter sets. The time-resolved PD could be as high as 50%, while the time-integrated one is roughly 17% in the optical band. The time-resolved PD in X-rays can reach 60% and the time-integrated one is around 30%–40%. The evolution of polarization angle (PA) is random in both optical and gamma-ray bands for the photosphere model, while it is roughly constant in the synchrotron models. Therefore, future time-resolved PA observations in the prompt optical or gamma-ray band could distinguish between the photosphere and synchrotron models. [ABSTRACT FROM AUTHOR]

The relativistic outflows that produce long gamma-ray bursts (LGRBs) can be described by a structured jet model where prompt |$\gamma$| -ray emission is restricted to a narrow region in the jet's core. Viewing the jet off-axis from the core, a population of afterglows without an associated GRB detection can be predicted. In this work, we conduct an archival search for these 'orphan' afterglows (OAs) with minute-cadence, deep (⁠|$g\sim 23$|⁠) data from the Dark Energy Camera (DECam) taken as part of the Deeper, Wider, Faster programme (DWF). We introduce a method to select fast-evolving OA candidates within DWF data that comprises a machine learning model, based on a realistic synthetic population of OAs. Using this classifier, we recover 51 OA candidates. Of these candidates, 42 are likely flare events from M-class stars. The remaining nine possess quiescent, coincident sources in archival data with angular profiles consistent with a star and are inconsistent with the expected population of LGRB host galaxies. We therefore conclude that these are likely Galactic events. We calculate an upper limit on the rate of OAs down to |$g\lt 22$| AB mag of 7.46 deg |$^{-2}$| yr |$^{-1}$| using our criteria and constrain possible jet structures. We also place an upper limit of the characteristic angle between the |$\gamma$| -ray-emitting region and the jet's half-opening angle. For a smooth power law and a power law with core jet model, respectively, these values are |$58.3^{\circ }$| and |$56.6^{\circ }$|⁠ , for a power-law index of 0.8 and |$75.3^{\circ }$| and |$76.8^{\circ }$| for a power-law index of 1.2. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs), often attributed to massive star collapse or binary compact object mergers, exhibit diverse emission characteristics hinting at multiple GRB classes based on various factors like progenitors, radiation mechanisms, and central engines. This study employs unsupervised clustering using the nested Gaussian mixture model algorithm to analyze data from Fermi and BATSE, identifying four classes (A–D) based on duration, spectral peak, and spectral index of time-integrated spectra of GRBs. Class proportions are approximately 70%, 10%, 3%, and 17%, respectively, with A and B comprising mostly long GRBs, C mainly short GRBs, and D encompassing both types. The classes are further assessed based on spectral index α, indicating distinct radiation mechanisms: α > −0.67 for photospheric emission, α ≤ −1.5 for fast-cooling synchrotron, and −1.5 < α ≤ −0.67 for slow-cooling synchrotron. Classes B and C align with photospheric emission, while A and D predominantly exhibit synchrotron radiation. Short GRBs are predominantly photospheric emission, whereas long GRBs tend to favor synchrotron emission. Overall, 63% of the total bursts exhibit α profiles indicative of synchrotron emission, with the remaining 37% associated with photospheric emission. Considering the limited data of kilonova and supernova associated with GRBs, classes are examined for progenitor origins, suggesting a hybrid nature for A and D, and collapsar and merger origins for B and C, respectively. This clustering analysis results in four GRB classes, which, upon investigation, reveal the diverse and complex nature of GRBs in terms of their radiation, duration, and progenitor. [ABSTRACT FROM AUTHOR]

We present the semi-analytical light curve modelling of 13 supernovae associated with gamma-ray bursts (GRB-SNe) along with two relativistic broad-lined (Ic-BL) SNe without GRB association (SNe 2009bb and 2012ap), considering millisecond magnetars as central-engine-based power sources for these events. The bolometric light curves of all 15 SNe in our sample are well-regenerated utilizing a χ2-minimization code, MINIM , and numerous parameters are constrained. The median values of ejecta mass (M ej), magnetar's initial spin period (P i), and magnetic field (B) for GRB-SNe are determined to be ≈5.2 M⊙, 20.5 ms, and 20.1 × 1014 G, respectively. We leverage machine learning (ML) algorithms to comprehensively compare the three-dimensional parameter space encompassing M ej, P i, and B for GRB-SNe determined herein to those of H-deficient superluminous SNe (SLSNe-I), fast blue optical transients (FBOTs), long GRBs (LGRBs), and short GRBs (SGRBs) obtained from the literature. The application of unsupervized ML clustering algorithms on the parameters M ej, P i, and B for GRB-SNe, SLSNe-I, and FBOTs yields a classification accuracy of ∼95 per cent. Extending these methods to classify GRB-SNe, SLSNe-I, LGRBs, and SGRBs based on P i and B values results in an accuracy of ∼84 per cent. Our investigations show that GRB-SNe and relativistic Ic-BL SNe presented in this study occupy different parameter spaces for M ej, P i, and B than those of SLSNe-I, FBOTs, LGRBs, and SGRBs. This indicates that magnetars with different P i and B can give birth to distinct types of transients. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are intense and short-lived cosmic explosions. Miniaturised CubeSat-compatible instruments for the study of GRBs are being developed to help bridge the gap in large missions and assist in achieving full sky coverage. CubeSats are small, compact satellites conforming to a design standard and have transformed the space industry. They are relatively low-cost and are developed on fast timescales, which has provided unparalleled access to space. This paper focuses on GMOD, the gamma-ray module, onboard the 2U CubeSat EIRSAT-1, launched on December 1st 2023. GMOD is a scintillation-based instrument with a cerium bromide crystal coupled to an array of sixteen silicon photomultipliers, designed for the detection of GRBs. The characterisation of GMOD in the spacecraft, along with the validation of an updated spacecraft MEGAlib model is presented and this approach can be followed by other CubeSats with similar science goals. The energy resolution of the flight model is 7.07% at 662 keV and the effective area peaks in the tens to hundreds of keV, making it a suitable instrument for the detection of GRBs. An investigation into the instrument's angular response is also detailed. The results from this characterisation campaign are a benchmark for the instrument's performance pre-launch and will be used to compare with the detector's performance in orbit. [ABSTRACT FROM AUTHOR]

Despite being operational for only a short time, the Einstein Probe mission, with its large field of view and rapid localization capabilities, has already significantly advanced the study of rapid variability in the soft X-ray sky. We report the discovery of luminous and variable radio emission from the Einstein Probe fast X-ray transient EP240414a, the second such source with a radio counterpart. The radio emission at 3 GHz peaks at ∼30 days postexplosion and with a spectral luminosity ∼2 × 10 ^30 erg s ^−1 Hz ^−1 , similar to what is seen from long gamma-ray bursts, and distinct from other extragalactic transients including supernovae and tidal disruption events, although we cannot completely rule out emission from engine driven stellar explosions, e.g., the fast blue optical transients. An equipartition analysis of our radio data reveals that an outflow with at least a moderate bulk Lorentz factor (Γ ≳ 1.6) with a minimum energy of ∼10 ^48 erg is required to explain our observations. The apparent lack of a reported gamma-ray counterpart to EP240414a could suggest that an off-axis or choked jet could be responsible for the radio emission, although a low-luminosity gamma-ray burst may have gone undetected. Our observations are consistent with the hypothesis that a significant fraction of extragalactic fast X-ray transients are associated with the deaths of massive stars.

Gamma-ray bursts are known to display various features on top of their canonical behavior. In this short review, we will describe and discuss two of them: the ultra-long gamma-ray bursts, which are defined by an extreme duration of their prompt phase, and the plateau phase, which is defined by a steady phase of large duration at the start of the afterglow. We will review the main properties of those two phenomena, and will discuss their possible origin, in light of the standard fireball model of gamma-ray bursts. A final section will discuss the future missions, which could bring new evidence to the study of those objects.

The relativistic jets of gamma-ray bursts (GRBs) might be powered by a black-hole (BH) hyperaccretion system. The inherent asymmetry in these jets generates recoil forces, inducing oscillations and positional deviations of the BH from equilibrium. In this study, we explore the influence of different initial BH mass, spin, and mass accretion rate, as well as their evolutions on the dynamical properties of BH under the effect of asymmetric jets. Our results reveal that the initial mass and accretion rate significantly impact the BH’s acceleration, velocity, and displacement, while the different initial spin plays a negligible role in shaping the overall dynamical evolution. Additionally, we calculate the gravitational wave (GW) strains associated with the asymmetric jets, finding that the resulting GW signals are too weak to be detected, even for nearby GRBs. These findings provide critical insights into the dynamical response of BHs to asymmetric jets and the associated GW radiation, advancing our understanding of BH physics in GRBs.

Gamma-ray bursts (GRBs) are often referred to as the most luminous explosions in the Universe, due to their short and highly luminous prompt emission. This apparent luminosity, however, does not reflect the true energy budget of the prompt emission, which is strongly beamed. Accurate estimations of the energy radiated during the prompt phase require taking the geometry of GRB jets into account, which remains poorly known. Nevertheless, one may establish the distribution of well-measured quantities, like E _iso , the GRB isotropic equivalent energy, which encrypts crucial information about GRB jets, with the aim of providing constraints on the jet's radiated energy. In this work, we study the bright end of the GRB isotropic equivalent energy distribution (hereafter called “apparent energy”), using an updated sample of 185 apparently energetic GRBs with E _iso ≥ 10 ^53 erg. This new sample includes GRB 221009A, allowing us to discuss this apparently superenergetic GRB in the context of the general E _iso distribution of long GRBs. We describe the construction of the sample and compare three fits of the E _iso distribution with a simple power law, a cutoff power law, and a broken power law. Our study confirms the existence of a cutoff around E _iso = 4 × 10 ^54 erg, even when GRB 221009A is included in the sample. Based on this finding, we discuss the possible reasons behind the rapid decrease in the number of apparently energetic gamma-ray bursts beyond E _iso = 4 × 10 ^54 erg and the interpretation of GRB 221009A, the most apparently energetic GRB detected to date, in this context.

Magnetar giant flares (MGFs) are the most energetic non-catastrophic transients known to originate from stellar objects. The first discovered events were nearby. In recent years, several extragalactic events have been identified, implying an extremely high volumetric rate. We show that future instruments with a sensitivity ≲5 × 10 ^−9 erg cm ^−2 at ∼1 MeV will be dominated by extragalactic MGFs over short gamma-ray bursts (sGRBs). Clear discrimination of MGFs requires intrinsic GRB localization capability to identify host galaxies. As MGFs involve a release of a sizable fraction of the neutron star's magnetic free energy reservoir in a single event, they provide us with invaluable tools for better understanding magnetar birth properties and the evolution of their magnetic fields. A major obstacle is to identify a (currently) small subpopulation of MGFs in a larger sample of more energetic and distant sGRBs. We develop the tools to analyze the properties of detected events and their occurrence rate relative to sGRBs. Even with the current (limited) number of events, we can constrain the initial internal magnetic field of a typical magnetar at formation to be B _0 ≈ 4 × 10 ^14 –2 × 10 ^15 G. Larger samples will constrain the distribution of birth fields. We also estimate the contribution of MGFs to the gravitational-wave (GW) stochastic background. Depending on the acceleration time of baryon-loaded ejecta involved in MGFs, their GW emission may reach beyond 10 kHz and, if so, will likely dominate over other conventional astrophysical sources in that frequency range.

The determination of the equation of state (EOS) of a neutron star (NS) and its maximum mass is very important for understanding the formation and properties of NSs under extreme conditions, but they remain open questions. Short-duration gamma-ray bursts (GRBs) are believed to originate from the merger of binary NSs or giant flares (GFs) of soft gamma repeaters (SGRs). Recently, the high-frequency quasi-periodic oscillations (QPOs) have been claimed to be identified from two short GRBs (GRB 931101B and GRB 910711). In this paper, we propose that the observed high-frequency QPOs in these two short GRBs result from torsional oscillations in the GFs of SGRs associated with cold NSs, or from radial oscillations of hypermassive NSs as the hot remnants of binary NS mergers, and then to constrain the EOS of NSs. For torsional oscillations, the six selected EOSs (TM1, NL3, APR, SLy4, DDME2, and GM1) of NSs suitable for the zero-temperature condition exhibit significant overlap in mass ranges, suggesting that we cannot constrain the EOS of NSs. For radial oscillations, the six selected EOSs (IUF, TM1, TMA, FSG, BHBLp, and NL3) of NSs suitable for the high-temperature condition cannot be ruled out when redshift is considered. However, it is found that the EOS can only be constrained if the redshift and temperature of the remnant can be measured.

Gamma-ray bursts (GRBs) are widely suggested as potential sources of ultrahigh-energy cosmic rays. The kinetic energy of the jets dissipates, leading to the production of an enormous amount of gamma-ray photons and possibly also the acceleration of protons. The accelerated protons will interact with the radiation of the GRB via the photomeson and Bethe–Heitler processes, which can initiate electromagnetic cascades. This process can give rise to broadband radiation up to the GeV–TeV gamma-ray regime. The expected gamma-ray flux from cascades depends on the properties of the GRB jet, such as the dissipation radius R _diss , the bulk Lorentz factor Γ, and the baryon loading factor η _p . Therefore, observations of Fermi-LAT can impose constraints on these important parameters. In this study, we select 12 GRBs of high keV–MeV fluence and constrain the baryon loading factor, under different combinations of the bulk Lorentz factor and the dissipation radius based on Fermi-LAT's measurements. Our findings indicate a strong constraint of η _p < 10 for most selected GRBs over a large parameter space except for large dissipation radii (≳10 ^15 cm) and high bulk Lorentz factors (≳600). The constraint is comparable to, and in some GRBs even stronger than, that from high-energy neutrinos for stacked GRBs. Our results suggest that for a typical bulk Lorentz factor of several hundred, the dissipation radii of GRBs need to be large to avoid overshooting the GeV gamma-ray flux during the prompt emission phase of GRBs, which can be used to constrain GRBs.

The duration of more than one thousand gamma-ray bursts (GRBs) was measured by using the Swift satellite. Besides the redshift distribution of GRBs, the burst duration is another significant property of GRBs that can be analyzed. In this paper, we first find the detection rate of Swift/Burst Alert Telescope for a cosmological model with the ω CDM model. Then, by performing a Monte Carlo simulation, we find the "long" GRB duration histogram to compare the cosmological model with the ω CDM model and the ΛCDM model. The χ ^2 minimization method is employed to determine the optimal value of α in the dark energy equation of state, P = cω ( z ) ρ , with ω ( z ) = −1 + α z. We show that for the ωCDM model, all values of α within α = [0, 0.16] range are considered acceptable within two standard deviations.

Polarization spectra were predicted within the photosphere model. To seek more clues to distinguish between the models, both the time-resolved and time-integrated polarization spectra from optical band to MeV gamma-rays of the magnetic reconnection model are studied here. There are two newly found differences between the two models. First, the time-integrated polarization degree of the magnetic reconnection model would in general increase with frequency for on-axis observations, while it is not monotonous for the photosphere model. Second, the variations of both the time-integrated and the time-resolved polarization angles (PAs) with frequency of the magnetic reconnection model are not random, while the time-integrated PA varies randomly with frequency for the photosphere model. Therefore, future energy-resolved polarization analysis could distinguish between the two models. In addition, the PA rotation spectra are studied for the first time. The rotation value of PA within the burst duration will decrease with the increase of the observational energy band. Most significant PA rotation would happen for slightly off-axis observations in each energy band. The PA would rotate even for on-axis observations in the optical band. Compared with the aligned magnetic field case, the PA rotation is quite rare in the gamma-ray band for the case with a toroidal field in the radiation region.

The X-ray afterglow of a certain fraction of gamma-ray bursts (GRBs) comprises a phase of shallow decay. However, the origin of the shallow decay segment is still a mystery. In this paper, we select 103 GRBs that exhibit a shallow decay segment, based on Swift/XRT data prior to 2023 June. We apply the fallback accretion energy external dissipation model to the shallow decay segment in our sample, and find the model can interpret the shallow decay segment within a reasonable range of parameter space. The fallback rate of the progenitor envelope materials onto the accretion disk can reach a peak of 10 ^−7 –0.1 M _⊙ s ^−1 within a time frame of 10–10 ^5 s, which is consistent with the typical density profile and spin profile of the progenitor of a GRB. During the fallback process of progenitor envelope materials, progenitor stars of most GRBs have the highest fallback rate at a radius ~10 ^10 cm, which is consistent with the typical radius of a Wolf–Rayet star. The production of shallow decay segments of most GRBs in our sample requires injecting Blandford–Znajek (BZ) energy equal to 10–100 times the kinetic energy of the initial GRB blast wave into the blast wave. However, the shallow decay segment of a few GRBs requires the injected energy to be comparable to the kinetic energy of the blast wave, which means that even if less energy is injected into a GRB blast wave, it is still possible to produce a shallow decay segment. In addition, the peak time of the fallback rate of the progenitor envelope materials is close to the time when BZ energy is injected into the blast wave, which means that the fallback accretion is more active in the initial phase. In summary, our research results provide further support to an external origin for the shallow decay segment in GRBs.

We present uniform modeling of eight kilonovae, five following short gamma-ray bursts (GRBs; including GRB 170817A) and three following long GRBs. We model their broadband afterglows to determine the relative contributions of afterglow and kilonova emission. We fit the kilonovae using a three-component model in MOSFiT , and report population median ejecta masses for the total, blue ( κ _B = 0.5 cm ^2 g ^−1 ), purple ( κ _P = 3 cm ^2 g ^−1 ), and red ( κ _R = 10 cm ^2 g ^−1 ) components. The kilonova of GW170817 is near the sample median in most derived properties. We investigate trends between the ejecta masses and the isotropic-equivalent and beaming-corrected γ -ray energies ( E _γ _,iso , E _γ ), as well as rest-frame durations ( T _90,rest ). We find long GRB kilonovae have higher median red ejecta masses ( M _ej,R ≳ 0.05 M _⊙ ) compared to on-axis short GRB kilonovae ( M _ej,R ≲ 0.02 M _⊙ ). We also observe a weak scaling between the total and red ejecta masses with E _γ _,iso and E _γ , though a larger sample is needed to establish a significant correlation. These findings imply a connection between merger-driven long GRBs and larger tidal dynamical ejecta masses, which may indicate that their progenitors are asymmetric compact object binaries. We produce representative kilonova light curves, and find that the planned depths and cadences of the Rubin and Roman Observatory surveys will be sufficient for order-of-magnitude constraints on M _ej,B (and, for Roman, M _ej,P and M _ej,R ) of future kilonovae at z ≲ 0.1.

SIRI-2 is a collection of strontium iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pileup corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with those from the unsaturated Insight and GECAM instruments and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 ± 0.002 erg cm ^−2 , lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4000 yr.

GRB 191019A was a long gamma-ray burst (GRB) lasting ~65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy ( z = 0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus reanalyzed unpublished GROND multicolor ( $g^{\prime} r^{\prime} i^{\prime} z^{\prime} JH{K}_{s}$ ) data obtained between 0.4 and 15 days posttrigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days postburst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of ~0.06 M _⊙ . Contrary to previous findings inferring a high-density circumburst environment ( n _0 ~ 10 ^7−8 cm ^−3 ), our analysis finds standard conditions ( n _0 ~ 1 cm ^−3 ), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.

Gamma-ray bursts (GRBs) are considered the most violent explosions in the Universe. Numerous studies have analyzed the formation rate of long gamma-ray bursts (LGRBs), which originate from the core collapse of massive stars, in comparison with the star formation rate (SFR). It is found that the LGRB formation rate at redshift z < 1 is significantly distinct from the SFR and is higher. Moreover, due to the threshold of detectors, the structured jets of GRBs also affect detection numbers at different redshifts, which all influence the calculation of the formation rate. Thus we investigate the LGRB formation rate at low redshift and further explore the possible correlation between the LGRB formation rate under the influence of structural jets and the SFR. We use Lynden-Bell's c ^− method to study the luminosity function and rate of Swift LGRBs without any assumptions. After correcting for the redshift evolution, the luminosity function can be expressed as $\psi ({L}_{0})\propto {L}_{0}^{-0.17\pm 0.01}$ for dim LGRBs and $\psi ({L}_{0})\propto {L}_{0}^{-1.36\pm 0.05}$ for bright LGRBs, and the formation rate at z < 1 shows an increase. Then we use Monte Carlo simulations to create pseudo-GRBs with structured jets and calculate the formation rates assuming that the actual LGRB formation rate at low redshift is proportional to the SRF. The results demonstrate that structural jets increase the number of detections at low redshift in response to observational effects. Compared with the observed formation rate, the structured jet model with Gauss-cocoon is found to be relatively more efficient, and the parameters η of this model are limited and analyzed to [0.5, 5].

Type I gamma-ray bursts (GRBs) are believed to originate from compact binary mergers usually with a duration of main emission less than 2 s. However, recent observations of GRB 211211A and GRB 230307A indicate that some merger-origin GRBs could last much longer. Since they show strikingly similar properties (indicating a common mechanism), which are different from the classic “long”-short burst (e.g., GRB 060614), we find they form an interesting subclass of type I GRBs, and we suggest to name them as type IL GRB. We find that the prompt emission of type IL GRB is composed of three episodes: (1) a precursor followed by a short quiescent (or weak emission) period, (2) a long-duration main emission, and (3) an extended emission. With this burst pattern, a good candidate, GRB 170228A, was found in the Fermi/Gamma-ray Burst Monitor archive data. Temporal and spectral analyses indeed show that GRB 170228A falls in the same group with GRB 211211A and GRB 230307A in many diagnostic figures. Thus, this burst pattern could be a good reference for rapidly identifying type IL GRBs and very helpful for conducting low-latency follow-up observation. We estimated the occurrence rate and discussed the physical origins and implications for the three emission episodes of type IL GRBs. Our analysis suggests the premerger precursor model, especially the magnetar super flare model, is more favored for type IL GRBs. More observations in multiwavelength and multimessenger are required to deepen our understanding of this subclass of GRB.

Gamma-ray bursts (GRBs) are the most luminous transients in the Universe. The interaction of the relativistic jet with the circumburst medium produces an afterglow and generates multiwavelength emission. In this work, we present simultaneous multiband photometry of GRB 240825A with the Multi-channel Photometric Survey Telescope (Mephisto) and analyze its temporal and spectral properties. The measurement began 128 s after the GRB trigger and continued until the fourth day, when the afterglow essentially diminished and the measured brightness was close to that of the host galaxy. Based on the multiband light curves in the uvgriz bands, we find that the optical flux density satisfies F _ν _,obs ∝ t ^−1.34 ν ^−2.48 with a spectral index of 2.48, much larger than those of most other GRBs. To reconcile the measured much softer spectral energy distribution (SED) with that predicted by the standard afterglow model, an extra host-galaxy extinction of E _B _− _V ∼ (0.37–0.57) mag is required. We interpreted this excess as arising from a dense circumburst medium. We further find that the SED of the optical afterglow hardened as the afterglow decayed and the color excess E _B _− _V decreased ∼0.26 mag from 100 to 3000 s after the GRB trigger. Finally, we analyze the properties of the host galaxy of GRB 240825A based on data from the Sloan Digital Sky Survey, the Pan-STARRS survey, and the HSC-SSP survey. For a host redshift of z = 0.659, the stellar mass and star formation rate of the host galaxy are estimated to be $\mathrm{log}({M}_{* }/{M}_{\odot })=10.{0}_{-0.3}^{+0.3}$ and $\mathrm{log}({\rm{SFR}}/{M}_{\odot }{{\rm{yr}}}^{-1})=0.{6}_{-3.3}^{+0.8}$ , respectively, pointing to a gas-rich, star-forming, medium-size galaxy.

The merger of compact binary stars produces short gamma-ray bursts (sGRBs), involving channels such as neutron star–neutron star (BNS) and neutron star–black hole (NS–BH). The association between sGRB 170817A and gravitational wave GW170817 provides reliable evidence for the BNS channel. Some speculations suggest that sGRBs with extended emission (EE) may represent another distinct population. The offset is the distance between the GRB sky localization and the host galaxy center. We compared the offset distributions of these two types of samples (46 sGRBs with EE and 9 without EE samples) and found that they follow the same distribution. Utilizing nonparametric methods, we examined the luminosity function and formation rate of sGRBs without any assumptions. The luminosity function can be described as $\psi ({L}_{0})\propto {L}_{0}^{-0.12\pm 0.01}$ for ${L}_{0}\lt {L}_{0}^{b}$ ( $\psi ({L}_{0})\propto {L}_{0}^{-0.73\pm 0.02}$ and for ${L}_{0}\gt {L}_{0}^{b}$ ) for sGRBs without EE and $\psi ({L}_{0})\propto {L}_{0}^{-0.13\pm 0.003}$ for ${L}_{0}\lt {L}_{0}^{b}$ ( $\psi ({L}_{0})\propto {L}_{0}^{-0.61\pm 0.01}$ and for ${L}_{0}\gt {L}_{0}^{b}$ ) for sGRBs with EE. The formation rate is characterized as ρ ( z ) ∝ (1 + z ) ^−3.04±0.10 for z < 1 and as ρ ( z ) ∝ (1 + z ) ^−0.29±0.38 for 1 < z < 3 for sGRBs without EE, while for sGRBs with EE, it is ρ ( z ) ∝ (1 + z ) ^−3.85±0.15 for z < 1 and ρ ( z ) ∝ (1 + z ) ^−0.40±1.11 for 1 < z < 3. Our findings suggest no significant difference in the progenitors of sGRBs with and without EE when considered in terms of spatial offsets, formation rates, and luminosity function.

The Low-Energy X-ray Polarization Detector (LPD) is one of the payloads in the POLAR-2 experiment, which is planned as an external payload for deployment on the China Space Station in 2026. The LPD is specifically designed to observe the polarization of gamma-ray burst prompt emissions in the energy range of 2–10 keV, with a preliminary design featuring a wide field-of-view (FOV) of ±45°. This observation is achieved using an array of X-ray photoelectric polarimeters. Because of the wide FOV design of the detector, nearly all photons impinge on the detector off-axis. This paper investigates the polarization reconstruction of a photoelectric polarimeter for off-axis photons. It was found that the reconstruction of the emission azimuthal angles of photoelectron tracks is the main factor affecting the shape of the modulation function for off-axis radiation. Therefore, a modulation function correction method based on Monte Carlo (MC) simulation is proposed. This MC simulation-based method is applied to both simulated and experimental data to correct the modulation function for off-axis radiation. The reconstruction results demonstrate that this method can accurately determine the polarization degree and polarization angle of photons under different conditions. While we tested up to 45° off-axis angles due to our ±45° FOV, the method is theoretically applicable to off-axis angles up to ∼90°.

Neutron star mergers are astrophysical "gold mines," synthesizing over half of the elements heavier than iron through rapid neutron capture nucleosynthesis. The observation of the binary neutron star merger GW170817, detected both in gravitational waves and electromagnetic radiation, marked a breakthrough. One electromagnetic component of this event, the gamma ray burst GRB 170817A, has an unresolved aspect: the characteristics of its prompt gamma-ray emission spectrum. In this work, we investigate that gamma-ray spectra in such GRBs may be influenced by de-excitations from isomeric transitions. Our study begins with a review of current knowledge on GRB structure and of r-process nucleosynthesis in neutron star collisions, focusing on the role of nuclear isomers in these settings. We then test our hypothesis by developing criteria to select representative isomers, based on known solar element abundances, for modeling GRB spectral characteristics. We integrate these criteria into an interactive web page, facilitating the construction and analysis of relevant gamma-ray spectra from isomeric transitions. Our analysis reveals that three isomers -- 90Zr, 207Pb, and 89Y -- stand out for their potential to impact the prompt GRB spectrum due to their specific properties. This information allows us to incorporate nuclear isomer data into astrophysical simulations and calculate isomeric abundances generated by astrophysical r-processes in neutron star mergers and their imprint on the detected signal. [ABSTRACT FROM AUTHOR]

Fulfilling the rich promise of rapid advances in time-domain astronomy is only possible through confronting our observations with physical models and extracting the parameters that best describe what we see. Here, we introduce redback ; a Bayesian inference software package for electromagnetic transients. redback provides an object-orientated python interface to over 12 different samplers and over 100 different models for kilonovae, supernovae, gamma-ray burst afterglows, tidal disruption events, engine-driven transients among other explosive transients. The models range in complexity from simple analytical and semi-analytical models to surrogates built upon numerical simulations accelerated via machine learning. redback also provides a simple interface for downloading and processing data from various catalogues such as Swift and FINK. The software can also serve as an engine to simulate transients for telescopes such as the Zwicky Transient Facility and Vera Rubin with realistic cadences, limiting magnitudes, and sky coverage or a hypothetical user-constructed survey or a generic transient for target-of-opportunity observations with different telescopes. As a demonstration of its capabilities, we show how redback can be used to jointly fit the spectrum and photometry of a kilonova, enabling a more powerful, holistic probe into the properties of a transient. We also showcase general examples of how redback can be used as a tool to simulate transients for realistic surveys, fit models to real, simulated, or private data, multimessenger inference with gravitational waves, and serve as an end-to-end software toolkit for parameter estimation and interpreting the nature of electromagnetic transients. [ABSTRACT FROM AUTHOR]

Ion Weibel instability is considered to be the dominant physics for the dissipation in high-Mach number astrophysical shocks such as supernova remnant shocks and gamma-ray burst shocks. We study the instability dependence on various parameters using theory and particle-in-cell simulations. We demonstrate that electron physics determines the saturation level of the Weibel-generated magnetic field, even though the instability is driven by the ions. We discuss the application to astrophysical and laboratory laser experiment environments to clarify the roles of the ion Weibel instability. We develop a model for the isotropization length-scale in Weibel-mediated shocks and compare its value to other characteristic length-scales of each system. We find that electron heating to near equipartition is crucial for the formation of ultrarelativistic Weibel-mediated shocks. On the other hand, our results imply that non-relativistic shocks in the typical interstellar medium are not purely mediated by the Weibel instability. [ABSTRACT FROM AUTHOR]

More than fifty years have elapsed from the first discovery of gamma-ray bursts (GRBs) with American Vela satellites, and more than twenty-five years from the discovery with the BeppoSAX satellite of the first X-ray afterglow of a GRB. Thanks to the afterglow discovery and to the possibility given to the optical and radio astronomers to discover the GRB optical counterparts, the long-time mystery about the origin of these events has been solved. Now we know that GRBs are huge explosions, mainly ultra relativistic jets, in galaxies at cosmological distances. Starting from the first GRB detection with the Vela satellites, I will review the story of these discoveries, those obtained with BeppoSAX, the contribution to GRBs by other satellites and ground experiments, among them being Venera, Compton Gamma Ray Observatory, HETE-2, Swift, Fermi, AGILE, MAGIC, H.E.S.S., which were, and some of them are still, very important for the study of GRB properties. Then, I will review the main results obtained thus far and the still open problems and prospects of GRB astronomy. [ABSTRACT FROM AUTHOR]

The TeV observations of GRB 221009A provided us with a unique opportunity to analyse the contemporaneous phase in which both prompt and afterglow emissions are seen simultaneously. To describe this initial phase of gamma-ray burst afterglows, we suggest a model for a blast wave with an intermittent energy supply. We treat the blast wave as a two-element structure. The central engine supplies energy to the inner part (shocked ejecta material) via the reverse shock. As the shocked ejecta material expands, its internal energy is transferred to the shocked external matter. We take into account the inertia of the shocked external material so that the pressure difference across this region determines the derivative of the blast wave's Lorentz factor. Applied to GRB 221009A, the model yields a very good fit to the observations of the entire TeV light curve except for three regions where there are excesses in the data with respect to the model. Those are well correlated with the three largest episodes of the prompt activity and thus, we interpret them as the reverse shock emission. Our best-fitting solution for GRB 221009A is an extremely narrow jet with an opening angle θ j ≈ 0.07° (500/Γ0) propagating into a wind-like external medium. This extremely narrow angle is consistent with the huge isotropic equivalent energy of this burst, and its inverse jet break explains the very rapid rise of the afterglow. Such inverse jet break occurs in an accelerating blast wave when the relativistic beaming becomes narrower than the jet's opening angle. Interestingly, photon–photon annihilation does not play a decisive role in the best-fitting model. [ABSTRACT FROM AUTHOR]

GRB 230812B is a bright and relatively nearby (z = 0.36) long gamma-ray burst (GRB) that has generated significant interest in the community and has thus been observed over the entire electromagnetic spectrum. We report over 80 observations in X-ray, ultraviolet, optical, infrared, and submillimetre bands from the GRANDMA (Global Rapid Advanced Network for Multimessenger Addicts) network of observatories and from observational partners. Adding complementary data from the literature, we then derive essential physical parameters associated with the ejecta and external properties (i.e. the geometry and environment) of the GRB and compare with other analyses of this event. We spectroscopically confirm the presence of an associated supernova, SN2023pel, and we derive a photospheric expansion velocity of v ∼ 17 × 103 km s−1. We analyse the photometric data first using empirical fits of the flux and then with full Bayesian inference. We again strongly establish the presence of a supernova in the data, with a maximum (pseudo-)bolometric luminosity of 5.75 × 1042 erg s−1, at |$15.76^{+0.81}_{-1.21}$|  d (in the observer frame) after the trigger, with a half-max time width of 22.0 d. We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Our best-fitting model favours a very low density environment (⁠|$\log _{10}({n_{\rm ISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}$|⁠) and small values for the jet's core angle |$\theta _{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm {deg}$| and viewing angle |$\theta _{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm {deg}$|⁠. GRB 230812B is thus one of the best observed afterglows with a distinctive supernova bump. [ABSTRACT FROM AUTHOR]

We consider the final evolutionary stages of a neutron star–black hole pair. According to the current paradigm, such systems eventually coalesce, which in some cases is accompanied by neutron-star tidal disruption. Using analytical methods, we show that the scenario of slow (of the order of several seconds) neutron-star stripping by the black hole is also possible, depending on the system parameters (the initial masses and intrinsic angular momenta of the components, the equation of state for the neutron star). Reaching the lower mass limit (about one tenth of the solar mass), the neutron star explodes to produce a comparatively powerful electromagnetic transient. Our population calculations show that the stripping mechanism is possible in 50–90 of the cases among all coalescing neutron star–black hole pairs, depending on the model assumptions about the evolution of close binary systems (the common-envelope efficiency parameter, the supernova explosion mechanism) and the initial metallicity of the stellar population. Because of the large mass of the ejected material, the kilonova emission in this scenario has good prospects of detection. [ABSTRACT FROM AUTHOR]

We propose that gamma-ray burst (GRB) pulses are produced when highly relativistic jets sweep across an observer's line of sight. We hypothesize that axisymmetric jet profiles, coupled with special relativistic effects, produce the time-reversed properties of GRB pulses. Curvature resulting from rapid jet expansion is responsible for much of the observed pulse asymmetry and hard-to-soft evolution. The relative obliqueness with which the jet crosses the line of sight explains the known GRB pulse morphological types. We explore two scenarios: one in which a rigid/semirigid jet moves laterally and another in which a ballistic jet sprays material from a laterally moving nozzle. The ballistic jet model is favored based upon its consistency with standard emission mechanisms. [ABSTRACT FROM AUTHOR]

In previous studies, the time evolution information is missed in deducing the time-integrated polarizations of gamma-ray burst prompt emission. Here, it is considered and the time-integrated polarizations is investigated through the summation of the time-resolved ones. The statistical properties of the distribution of the time-integrated polarization degree (Π) can be read from the q– Π curve, where q ≡ θ V /θ j . θ V and θ j are the observational and jet half-opening angles, respectively. Hence, only the q– Π curves are studied. In addition to a toroidal magnetic field in the radiation region, an aligned field is also discussed. We found the predicted time-integrated polarization degree is around |$(40\!-\!50)\,\, \mathrm{ per\,cent}$| for High-energy Polarimetry Detector (HPD) onboard POLAR-2 and is roughly |$(30\!-\!40)\,\, \mathrm{ per\,cent}$| for its Low-energy Polarimetry Detector (LPD). Therefore, Π value detected by the HPD will be larger than that of the LPD in statistics and the result of the former estimations will underestimate the value of Π in an ordered field. There are mainly two types of the q –Π curve profiles, corresponding to two ordered magnetic field configurations. [ABSTRACT FROM AUTHOR]

The properties of the progenitors of gamma-ray bursts and of their environment are encoded in their luminosity function and cosmic formation rate. They are usually recovered from a flux-limited sample based on Lynden-Bell's |$c^{-}$| method. However, this method is based on the assumption that the luminosity is independent of the redshift. Observationally, if correlated, people use non-parametric |$\tau$| statistical method to remove this correlation through the transformation, |$L^{\prime }=L/g(z)$|⁠ , where z is the burst redshift, and |$g(z)=(1+z)^{k}$| parametrizes the underlying luminosity evolution. However, the application of this method to different observations could result in very different luminosity functions. By the means of Monte Carlo simulation, in this paper, we demonstrate that the origin of an observed correlation, measured by the |$\tau$| statistical method, is a complex combination of multiple factors when the underlying data are correlated. Thus, in this case, it is difficult to unbiasedly reconstruct the underlying population distribution from a truncated sample, unless the detailed information of the intrinsic correlation is accurately known in advance. In addition, we argue that an intrinsic correlation between luminosity function and formation rate is unlikely eliminated by a misconfigured transformation, and the |$g(z)$|⁠ , derived from a truncated sample with the |$\tau$| statistical method, does not necessarily represent its underlying luminosity evolution. [ABSTRACT FROM AUTHOR]