Bright Gamma-ray Bursts Research Articles

The Thermal Emission in Short Gamma-Ray Bursts with Extended Emission Observed by Fermi/GBM

Short gamma-ray bursts (SGRBs) with extended emission (EE) are composed of initial main emission (ME) with a short hard spike, 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 of 36 gamma-ray bursts (GRBs) in our sample, which are identified as the category of SGRBs with EE as observed by Fermi/Gamma-ray Burst Monitor. By extracting time-integrated spectra of ME and EE with cutoff power-law or Band models for our sample, we find that 20 out of 36 SGRBs have α values that exceed the death line (e.g., −2/3) of synchrotron emission within either ME or EE phases, and we 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 nonthermal emissions do exist in the prompt emission of those four bright GRBs, which suggests that a hybrid jet (e.g., matter and Poynting-flux outflow) in GRBs 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 have been discovered. This indicates that the spectral evolution of both ME and EE seem to share similar behavior, possibly from the same physical origin.

A Millimeter Rebrightening in GRB 210702A

We present X-ray to radio frequency observations of the bright long gamma-ray burst GRB 210702A. Our Atacama Large Millimeter/submillimeter Array 97.5 GHz observations show a significant rebrightening by a factor of ≈2 beginning at 8.2 days post-burst and rising to peak brightness at 18.1 days before declining again. This is the first such rebrightening seen in a millimeter afterglow light curve. A standard forward shock model in a stellar wind circumburst medium can explain most of our X-ray, optical, and millimeter observations prior to the rebrightening, but significantly overpredicts the self-absorbed radio emission, and cannot explain the millimeter rebrightening. We investigate possible explanations for the millimeter rebrightening, and find that energy injection or a reverse shock from a late-time shell collision are plausible causes. Similar to other bursts, our radio data may require alternative scenarios such as a thermal electron population or a structured jet to explain the data. Our observations demonstrate that millimeter light curves can exhibit some of the rich features more commonly seen in optical and X-ray afterglow light curves, motivating further millimeter wavelength studies of GRB afterglows.

GRB 190114C: Fireball Energy Budget and Radiative Efficiency Revisited

The jet composition of gamma-ray bursts (GRBs), as well as how efficiently the jet converts its energy to radiation, are long-standing problems in GRB physics. Here, we reported a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its high fluence (∼4.4 × 10−4 erg cm−2) allows us to conduct the time-resolved spectral analysis in great detail and study their variations down to a very short timescale (∼0.1 s) while preserving a high significance. Its prompt emission consists of three well-separated pulses. The first two main pulses (P 1 and P 2) exhibit independently strong thermal components, starting from the third pulse (P 3) and extending to the entire afterglow, the spectra are all nonthermal, and the synchrotron plus Compton upscattering model well interprets the observation. By combining the thermal (P 1 and P 2) and the nonthermal (P 3) observations based on two different scenarios (global and pulse properties) and following the method described in Zhang et al., we measure the fireball parameters and GRB radiative efficiency with little uncertainties for this GRB. A relevantly high GRB radiative efficiency is obtained based on both the global and pulse properties, suggesting that if GRBs are powered by fireballs, the efficiency can sometimes be high. More interestingly, though the observed parameters are individually different (e.g., the amount of mass loading M), the radiative efficiency obtained from P 1 (η γ = 36.0% ± 6.5%) and P 2 (η γ = 41.1% ± 1.9%) is roughly the same, which implies that the central engine of the same GRB has some common properties.

Observational Evidence of Particle Acceleration by Relativistic Magnetic Reconnection in Gamma-Ray Bursts

Gamma-ray bursts (GRBs) as the most energetic explosions in the modern universe have been studied for over half a century, but the physics of the particle acceleration and radiation responsible for their observed spectral behaviors are still not well understood. Based on the comprehensive analysis of the pulse properties in both bright GRB 160625B and GRB 160509A, for the first time, we identify evidence of particle acceleration by relativistic magnetic reconnection from the evolutionary behavior of the two spectral breaks E p and E cut. Meanwhile, the adiabatic cooling process of the emitting particles in the magnetic reconnection regions produces a relation between the spectral index and the flux. We also discuss the physics behind spectral energy correlations. Finally, we argue that the identification of an anticorrelation between E cut and L iso may open a new avenue for diagnostics of the physics of the particle acceleration and radiation in a variety of astrophysical sources.

A Search for Soft X-Ray Emission Lines in the Afterglow Spectrum of GRB 221009A

GRB 221009A was the brightest gamma-ray burst (GRB) of all time (BOAT), surpassing in prompt brightness all GRBs discovered in ∼50 yr and in afterglow brightness in ∼20 yr. We observed the BOAT with XMM-Newton 2.3 days after the prompt. The X-ray afterglow was still very bright and we collected the largest number of photons with the reflection grating spectrometers (RGSs) on a GRB. We searched the RGS data for narrow emission or absorption features. We did not detect any bright line feature. A candidate narrow feature is identified at a (rest-frame) energy of 1.455−0.014+0.006 keV, consistent with an Mg xii K α emission line, slightly redshifted (0.012) with respect to the host galaxy. We assessed a marginal statistical significance of 3.0σ for this faint feature based on conservative Monte Carlo simulations, which requires caution for any physical interpretation. If this line feature would be for real, we propose that it might originate from the reflection in the innermost regions of the infalling funnel from low-level late-time activity emission of the central engine.

Exploring the Origin of Ultralong Gamma-Ray Bursts: Lessons from GRB 221009A

The brightest gamma-ray burst (GRB) ever, GRB 221009A, displays ultralong GRB (ULGRB) characteristics, with a prompt emission duration exceeding 1000 s. To constrain the origin and central engine of this unique burst, we analyze its prompt and afterglow characteristics and compare them to the established set of similar GRBs. To achieve this, we statistically examine a nearly complete sample of Swift-detected GRBs with measured redshifts. We categorize the sample to bronze, silver, and gold by fitting a Gaussian function to the log-normal of T 90 duration distribution and considering three subsamples respectively to 1, 2, and 3 times of the standard deviation to the mean value. GRB 221009A falls into the gold subsample. Our analysis of prompt emission and afterglow characteristics aims to identify trends between the three burst groups. Notably, the gold subsample (a higher likelihood of being ULGRB candidates) suggests a collapsar scenario with a hyperaccreting black hole as a potential central engine, while a few GRBs (GRB 060218, GRB 091024A, and GRB 100316D) in our gold subsample favor a magnetar. Late-time near-IR observations from 3.6 m Devasthal Optical Telescope rule out the presence of any bright supernova associated with GRB 221009A in the gold subsample. To further constrain the physical properties of ULGRB progenitors, we employ the tool MESA to simulate the evolution of low-metallicity massive stars with different initial rotations. The outcomes suggest that rotating (Ω ≥ 0.2 Ωc) massive stars could potentially be the progenitors of ULGRBs within the considered parameters and initial inputs to MESA.

Search for neutrino emission from GRB 221009A using the KM3NeT ARCA and ORCA detectors

Gamma-ray bursts are promising candidate sources of high-energy astrophysical neutrinos. The recent GRB 221009A event, identified as the brightest gamma-ray burst ever detected, provides a unique opportunity to investigate hadronic emissions involving neutrinos. The KM3NeT undersea neutrino detectors participated in the worldwide follow-up effort triggered by the event, searching for neutrino events. In this paper, we summarize subsequent searches, in a wide energy range from MeV up to a few PeVs. No neutrino events are found in any of the searches performed. Upper limits on the neutrino emission associated with GRB 221009A are computed.

A mega-electron volt emission line in the spectrum of a gamma-ray burst.

A long gamma-ray burst (GRB) is observed when the collapse of a massive star produces an ultrarelativistic outflow pointed toward Earth. Gamma-ray spectra of long GRBs are smooth, typically modeled by joint power-law segments describing a continuum, with no detected spectral lines. We report a significant (>6σ) narrow emission feature at ~10 mega-electron volts (MeV) in the spectrum of the bright GRB 221009A. Over 80 seconds, it evolves in energy (~12 to ~6 MeV) and in luminosity (~1.1 to <0.43 × 1050 erg second-1) but has a constant width of ~1 MeV. We interpret this feature as a blueshifted spectral line produced by the annihilation of electron-positron pairs, potentially in the same location responsible for emitting the brightest GRB pulses.

Multiband analyses of the bright GRB 230812B and the associated SN2023pel

ABSTRACT 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.

What Powered the Kilonova-like Emission after GRB 230307A in the Framework of a Neutron Star–White Dwarf Merger?

The second brightest gamma-ray burst, GRB 230307A (with a duration T 90 ∼ 40 s), exhibited characteristics indicative of a magnetar engine during the prompt emission phase. Notably, a suspected kilonova was identified in its follow-up optical and infrared observations. Here we propose that the origin of GRB 230307A is a neutron star–white dwarf (NS–WD) merger as this could naturally explain the long duration and the large physical offset from the center of its host galaxy. In the framework of such an NS–WD merger event, the late-time kilonova-like emission is very likely to be powered by the spin-down of the magnetar and the radioactive decay of 56Ni, rather than by the decay of r-process elements as these heavy elements may not easily be synthesized in an NS-WD merger. It is demonstrated that the above scenario can be supported by our fit to the late-time observational data, where a mass of ∼10−3 M ⊙ 56Ni is involved in the ejecta of a mass of ∼0.1 M ⊙. Particularly, the magnetar parameters required by the fit are consistent with those derived from the early X-ray observation.

The Case of the Missing Very High-energy Gamma-Ray Bursts: A Retrospective Study of Swift Gamma-Ray Bursts with Imaging Atmospheric Cherenkov Telescopes

Gamma-ray bursts (GRBs) are particle acceleration sites that can emit photons in the very high-energy (VHE) domain through nonthermal processes. From 2004 until 2018, the current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) did not detect any GRB in the VHE domain. However, from 2018 to 2020, five detections were reported. In this work, we try to solve the case of the missing VHE GBRs prior to 2018. We aim to identify GRBs that might have eluded VHE detection in past years by the H.E.S.S., MAGIC, and VERITAS IACTs. To do so, we study GRBs with known redshift detected by Swift from 2004 until June 2022. We first identify all GRBs that could have been observed by these IACTs since 2004, considering observation conditions and visibility constraints. We assume a relation between the X-rays and the VHE gamma rays based on the VHE GRBs detected to date and combine this with the redshift measurements, instrument response information, and observation conditions to predict the observed VHE gamma-ray flux from the Swift X-Ray Telescope measurements. We report findings on 12 bright low-redshift GRBs that we identify as most likely to have been detected in the VHE domain by current IACTs. The rate of IACT-detectable GRBs with ideal observation conditions is <1 VHE GRB per year with the current configuration. With its lower energy threshold and higher sensitivity, this rate increases to ∼4 VHE GRBs per year with the Cherenkov Telescope Array.

Evidence for a Compact Stellar Merger Origin for GRB 230307A From Fermi-LAT and Multiwavelength Afterglow Observations

GRB 230307A is the second-brightest gamma-ray burst (GRB) ever detected over 50 yr of observations and has a long duration in the prompt emission. Two galaxies are found to be close to the position of GRB 230307A: (1) a distant (z ∼ 3.87) star-forming galaxy, located at an offset of ∼0.″2–0.″3 from the GRB position (with a projected distance of ∼1–2 kpc); (2) a nearby (z = 0.065) spiral galaxy, located at an offset of 30″ (with a projected distance of ∼40 kpc). Though it has been found that the brightest GRBs are readily detected in GeV emission by the Fermi Large Area Telescope, we find no GeV afterglow emission from GRB 230307A. Combining this with the optical and X-ray afterglow data, we find that a circumburst density as low as ∼10−5–10−4 cm−3 is needed to explain the nondetection of GeV emission and the multiwavelength afterglow data, regardless of the redshift of this GRB. Such a low-density disfavors the association of GRB 230307A with the high-redshift star-forming galaxy, since the proximity of the GRB position to this galaxy would imply a higher-density environment. Instead, the low-density medium is consistent with the circumgalactic medium, which agrees with the large offset between GRB 230307A and the low-redshift galaxy. This points to the compact stellar merger origin for GRB 230307A, consistent with the detection of an associated kilonova.

Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts

The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor. A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. First, the parameter distributions of the time-resolved spectrum are given as follows: the low-energy spectral index α ∼ − 0.72, high-energy spectral index β ∼ − 2.42, the peak energy E p ∼ 221.69 keV, and the energy flux F ∼ 7.49 × 10−6 erg cm−2 s−1. More than 80% of the bursts exhibit the hardest low-energy spectral index αmax exceeding the synchrotron limit (−2/3). Second, the evolution patterns of α and E p were statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of both E p and α. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation between F and E p, with half of the samples exhibiting a positive correlation between F and α. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered.

Towards a model of photon-axion conversion in the host galaxy of GRB 221009A

GRB 221009A was the brightest gamma-ray burst ever detected on Earth. In its early afterglow phase, photons with exceptional energies above 10 TeV were observed by LHAASO, and a photon-like air shower above 200 TeV was detected by Carpet-2. Gamma rays of very high energies can hardly reach us from the distant GRB because of pair production on cosmic background radiation. Though final results on the highest-energy photons from this GRB have not been published yet, a number of particle-physics solutions to this problem were discussed in recent months. One of the most popular ones invokes the mixing of photons with axion-like particles (ALPs). Whether this is a viable scenario, depends crucially on the magnetic fields along the line of sight, which are poorly known. Here, we use the results of recent Hubble Space Telescope observations of the host galaxy of GRB 221009A, combined with magnetic-field measurements and simulations for other galaxies, to construct a toy model of the host-galaxy magnetic field and to estimate the rate of the photon-axion conversion there. Thanks, in particular, to the exceptional edge-on orientation of the host galaxy, strong mixing appears to be natural, both for LHAASO and Carpet-2 energy bands, for a wide range of ALP masses m ≲ 10-5 eV and photon couplings g ≳ 10-11 GeV-1.

Feasibility of using 3D CZT drift strip detectors for small Compton camera space missions

This feasibility study explores the possibility of using 3D CZT drift strip detectors developed by DTU Space in a small Compton camera payload, with the primary objective of technology demonstration. We have defined a scalable mass model for the payload, comprising eight 3D CZT drift strip detectors surrounded by CsI scintillator detectors for active shielding. The payload's angular resolution, effective area, and efficiency are evaluated through simulations of far-field monochromatic point sources. The instrument's sensitivity is assessed in a low Earth orbit background environment fornuclear line and continuum emission sources.With a 3σ point source sensitivity in the order of 10-4 [ph/cm2/s], it is evident that such an instrument only allows for limited scientific goals. In-orbit simulations of bright sources are conducted, resulting in reasonable observation times for the Crab Nebula at a 5σ significance level. Furthermore, in-orbit simulations of a selection of bright gamma-ray bursts indicate the potential for observing bright transient events. The study underscores the potential of using 3D CZT drift strip detectors in Compton camera configurations but also highlights the need for a larger effective area to improve sensitivity. However, for a technology demonstration aimed at increasing the Technology Readiness Level of the 3D CZT drift strip detector, a small Compton camera configuration like the one presented in this study could be a viable solution.

GRB 221009A, Its Precursor, and Two Afterglows in the Fermi Data

We study GRB 221009A, the brightest gamma-ray burst in the history of observations, using Fermi data. To calibrate them for large inclination angles, we use the Vela X gamma-ray source. Light curves in different spectral ranges demonstrate a 300 s overlap of afterglow and delayed episodes of soft prompt emission. We demonstrate that a relatively weak burst precursor that occurs 3 min before the main episode has its own afterglow, i.e., presumably, its own external shock. This is the first observation of such phenomenon which rules out some theoretical models of GRB precursors. The main afterglow is the brightest one, includes a photon with an energy of 400 GeV 9 h after the burst, we show that it is visible in the LAT data for up to two days.

Exploring Physically Motivated Models to Fit Gamma-Ray Burst Spectra

We explore fitting gamma-ray burst (GRB) spectra with three physically motivated models, and thus revisit the viability of synchrotron radiation as the primary source of GRB prompt emission. We pick a sample of 100 bright GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM), based on their energy flux values. In addition to the standard empirical spectral models used in previous GBM spectroscopy catalogs, we also consider three physically motivated models; (a) a thermal synchrotron model, (b) a Band model with a high-energy cutoff, and (c) a smoothly broken power-law (SBPL) model with a multiplicative broken power law (MBPL). We then adopt the Bayesian information criterion to compare the fits obtained and choose the best model. We find that 42% of the GRBs from the fluence spectra and 23% of GRBs from the peak-flux spectra have one of the three physically motivated models as their preferred one. From the peak-flux spectral fits, we find that the low-energy index distributions from the empirical model fits for long GRBs peak around the synchrotron value of −2/3, while the two low-energy indices from the SBPL+MBPL fits of long GRBs peak close to the −2/3 and −3/2 values expected for a synchrotron spectrum from marginally fast-cooling electrons.

On the “Loose” Constraint from IceCube Neutrino Nondetection of GRB 230307A

The recent extremely bright gamma-ray burst (GRB) from a binary neutron star merger, GRB 230307A, may offer a good probe for the production of GRB neutrinos. Within the constraint from IceCube neutrino nondetection, the limits for key physical parameters of this burst are extracted in different scenarios, including the fireball, Poynting-flux-dominated, and hybrid jet. Different from the former nearby “monsters” and due to its smaller isotropic equivalent radiated energy (E γ,iso ∼ 4 × 1052 erg), the constraint seems loose if nonthermal neutrinos produced from photomeson interactions are the only consideration. However, a quasi-thermal neutrino emission from hadronuclear processes is constrained in this neutron-rich postmerger environment, and the upper limit of the allowed nucleon loading factor is ∼a few. Based on this, a discussion is presented on the possible prompt emission mechanism and jet composition for GRB 230307A in the context of multimessenger astrophysics. It is worth noting that until now, no GRB neutrinos have been ever detected, even for the two brightest nearby GRBs ever observed (GRB 221009A and GRB 230307A), which have different dissipation mechanisms.

GRB 221009A with an Unconventional Precursor: A Typical Two-stage Collapsar Scenario?

As the brightest gamma-ray burst (GRB) ever detected, GRB 221009A may offer a chance that reveals some interesting features that are hidden in those bursts that are not so bright. There seems to be a very weak emission with a flux of 10−8 ∼ 10−7 erg cm−2 s−1 between the first pulse (T 0 ∼ T 0 + 50 s; T 0 is the trigger time) and the main burst (which appears from T 0 + 180 s). Thus, the gap time between them is not really quiescent, and the first pulse could be taken as an unconventional precursor, which may provide a peculiar case study for GRB precursor phenomena. A two-stage collapsar scenario is proposed as the most likely origin for this burst. In this model, the jet for the precursor is produced during the initial core-collapse phase, and should be weak enough not to disrupt the star when it breaks out of the envelope, so that the fallback accretion process and the forming of the disk could continue. We present an approach in which the duration and flux both provide constraints on the luminosity (L j) and the Lorentz factor at the breakout time (Γb) of this weak jet. The estimated L j ≲ 1049 erg s−1 and Γb has an order of 10, which are well consistent with the theoretical prediction. Besides, the weak emission in the gap time could be interpreted as an MHD outflow due to a magnetically driven wind during the period from the proto-neutron-star phase to the forming of the accretion disk in this scenario.

Heavy-element production in a compact object merger observed by JWST.

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs)1, sources of high-frequency gravitational waves (GWs)2 and likely production sites for heavy-element nucleosynthesis by means of rapid neutron capture (the r-process)3. Here we present observations of the exceptionally bright GRB 230307A. We show that GRB 230307A belongs to the class of long-duration GRBs associated with compact object mergers4-6 and contains a kilonova similar to AT2017gfo, associated with the GW merger GW170817 (refs. 7-12). We obtained James Webb Space Telescope (JWST) mid-infrared imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns, which we interpret as tellurium (atomic mass A = 130) and a very red source, emitting most of its light in the mid-infrared owing to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy-element nucleosynthesis across the Universe.