Bright Gamma-ray Bursts Research Articles

GRB 220408B: A Three-episode Burst from a Precessing Jet

Jet precession has previously been proposed to explain the apparently repeating features in the light curves of a few gamma-ray bursts (GRBs). In this paper, we further apply the precession model to a bright GRB 220408B by examining both its temporal and spectral consistency with the predictions of the model. As one of the recently confirmed GRBs observed by our GRID CubeSat mission, GRB 220408B is noteworthy as it exhibits three apparently similar emission episodes. Furthermore, the similarities are reinforced by their strong temporal correlations and similar features in terms of spectral evolution and spectral lags. Our analysis demonstrates that these features can be well explained by the modulated emission of a Fast-Rise-Exponential-Decay (FRED) shape light curve intrinsically produced by a precessing jet with a precession period of s, a nutation period of s and viewed off-axis. This study provides a straightforward explanation for the complex yet similar multi-episode GRB light curves.

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.

A Method to Measure Photometries of Moderately Saturated UVOT Sources

For bright transients such as gamma-ray bursts (GRBs), the Ultra-Violet/Optical Telescope (UVOT) operates under event mode at early phases, which records incident positions and the arrival time for each photon. The event file is able to be screened into many exposures to study the early light curve of GRBs with a high time resolution, including the rapid brightening of the UV/optical emission in particular. Such a goal, however, is hampered for some extremely bright GRBs by the saturation in UVOT event images. For moderately saturated UVOT sources, in this work we further develop the method proposed in Jin et al. to recover their photometries. The basic idea is to assume a stable point-spread function of UVOT images, for which the counts in the core region (i.e., an aperture of a radius of 5″) and the wing region (i.e., an annulus ranging from 15″–25″) should be constant, and the intrinsic flux can be reliably inferred with data in the ring. We demonstrate that in a given band, a tight correlation does hold among the background-removed count rates in the core and the wing. With the new method, the bright limit of measuring range for UVOT V and B bands increases by ∼1.7 mag, while only by ∼0.7 mag for the U band due to the lack of bright calibration sources. Systematic uncertainties are ∼0.2 mag for the V, B, and U bands.

GRBAlpha: The smallest astrophysical space observatory

Aims. Since it launched on 22 March 2021, the 1U-sized CubeSat GRBAlpha operates and collects scientific data on high-energy transients, making it the smallest astrophysical space observatory to date. GRBAlpha is an in-orbit demonstration of a gamma-ray burst (GRB) detector concept suitably small to fit into a standard 1U volume. As was demonstrated in a companion paper, GRBAlpha adds significant value to the scientific community with accurate characterization of bright GRBs, including the recent outstanding event of GRB 221009A. Methods. The GRB detector is a 75 × 75 × 5 mm CsI(Tl) scintillator wrapped in a reflective foil (ESR) read out by an array of SiPM detectors, multi-pixel photon counters by Hamamatsu, driven by two separate redundant units. To further protect the scintillator block from sunlight and protect the SiPM detectors from particle radiation, we applied a multi-layer structure of Tedlar wrapping, anodized aluminium casing, and a lead-alloy shielding on one edge of the assembly. The setup allows observations of gamma radiation within the energy range of 70–890 keV with an energy resolution of ~30%. Results. Here, we summarize the system design of the GRBAlpha mission, including the electronics and software components of the detector, some aspects of the platform, and the current semi-autonomous operations. In addition, details are given about the raw data products and telemetry in order to encourage the community to expand the receiver network for our initiatives with GRBAlpha and related experiments.

The peak flux of GRB 221009A measured with GRBAlpha

Context. On 2022 October 9 the brightest gamma-ray burst (GRB) ever observed lit up the high-energy sky. It was detected by a multitude of instruments, attracting the close attention of the GRB community, and saturated many detectors. Aims. GRBAlpha, a nano-satellite with a form factor of a 1U CubeSat, detected this extraordinarily bright long-duration GRB, GRB 221009A, without saturation but affected by pile-up. We present light curves of the prompt emission in 13 energy bands, from 80 keV to 950 keV, and performed a spectral analysis to calculate the peak flux and peak isotropic-equivalent luminosity. Methods. Since the satellite’s attitude information is not available for the time of this GRB, more than 200 incident directions were probed in order to find the median luminosity and its systematic uncertainty. Results. We find that the peak flux in the 80 − 800 keV range (observer frame) was Fphp = 1300−200+1200 ph cm−2 s−1, or Fergp = 5.7−0.7+3.7 × 10−4 erg cm−2 s−1, and the fluence in the same energy range of the first GRB episode, which lasted 300 s and was observable by GRBAlpha, was S = 2.2−0.3+1.4 × 10−2 erg cm−2, or Sbol = 4.9−0.5+0.8 × 10−2 erg cm−2 for the extrapolated range of 0.9 − 8690 keV. We infer the isotropic-equivalent released energy of the first GRB episode to be Eisobol = 2.8−0.5+0.8 × 1054 erg in the 1 − 10 000 keV band (rest frame at z = 0.15). The peak isotropic-equivalent luminosity in the 92 − 920 keV range (rest frame) was Lisop = 3.7−0.5+2.5 × 1052 erg s−1, and the bolometric peak isotropic-equivalent luminosity was Lisop,bol = 8.4−1.5+2.5 × 1052 erg s−1 (4 s scale) in the 1 − 10 000 keV range (rest frame). The peak emitted energy is Ep∗ = Ep(1+z) = 1120 ± 470 keV. Our measurement of Lisop,bol is consistent with the Yonetoku relation. It is possible that, due to the spectral evolution of this GRB and the orientation of GRBAlpha at the peak time, the true values of peak flux, fluence, Liso, and Eiso are even higher.

Self-gravitating collapsing star and black hole spin-up in long gamma ray bursts

Context. Long gamma ray bursts (GRBs) originate from the collapse of massive, rotating stars. Some of the GRBs exhibit much stronger variability patterns in the prompt GRB emission than the usual stochastic variations. We discuss the mechanisms able to account for this effect. Aims We aim to model the process of stellar collapse in the scenario of a self-gravitating collapsing star. We account for the changes in Kerr metric induced by the growth of the black hole; accretion of angular momentum; and the self-gravity effect due to a large mass of the collapsing stellar core falling onto black hole in a very short time. We also investigate the existence of accretion shocks in the collapsar, and the role of magnetic field in their propagation. Methods. We compute the time-dependent axially symmetric general relativistic magnetohydrodynamic model of a collapsing stellar core in the dynamical Kerr metric. We explore the influence of self-gravity in such a star, where the newly formed black hole is increasing the mass and changing its spin. The Kerr metric evolves according to the mass and angular momentum changes during the collapse. We parameterize the rotation inside the star, and account for the presence of large-scale poloidal magnetic field. For the set of the global parameters, such as the initial black hole spin and the initial content of specific angular momentum in the stellar envelope, we determine the evolution of black hole parameters (mass and spin) and quantify the strength of the gravitational instability. We then estimate the variability timescales and amplitudes. Results. We find that the role of the gravitational instability measured by the value of the Toomre parameter is relatively important in the innermost regions of the collapsing star. The character of accretion rate variability strongly depends on the assumption of self-gravity in the model, and is also affected by the magnetic field. Additional factors are initial spin and rotation of the stellar core. We find that for subcritical rotation of the precollapsed star, a centrifugally supported mini-disc is present at the equatorial plane, and it may be subject to fragmentation due to self-gravitating instability. We also find that self-gravity may play a role in the angular momentum transport and that it generally lowers the final mass and spin of the black hole, while the accretion-rate variability amplitude is much larger in self-gravitating objects. The effect of magnetic field is rather weak, while it seems to decrease the strength of accretion shocks. The magnetisation affects the global properties of the flow in a non-linear way, and is manifested mostly in models with moderate initial black hole spins but supercritical rotation of the collapsing star. Conclusions. Our computations confirm that gravitational instability can account for flaring activity in GRBs and the variations in their prompt emission. Rapid variability detected in the brightest GRBs (most likely powered by rapidly spinning black holes) is consistent with the self-gravitating collapsar model, where the transonic shocks are formed. The effect should be weakened by magnetic field.

Understanding the nature of the optical emission in gamma-ray bursts: analysis from TAROT, COATLI, and RATIR observations

ABSTRACT We collected the optical light-curve data of 227 gamma-ray bursts (GRBs) observed with the TAROT, COATLI, and RATIR telescopes. These consist of 133 detections and 94 upper limits. We constructed average light curves in the observer and rest frames in both X-rays (from Swift/X-Ray Telescope) and the optical. Our analysis focused on investigating the observational and intrinsic properties of GRBs. Specifically, we examined observational properties, such as the optical brightness function of the GRBs at T = 1000 s after the trigger, as well as the temporal slope of the afterglow. We also estimated the redshift distribution for the GRBs within our sample. Of the 227 GRBs analysed, we found that 116 had a measured redshift. Based on these data, we calculated a local rate of ρ0 = 0.2 Gpc−3 yr−1 for these events with z < 1. To explore the intrinsic properties of GRBs, we examined the average X-ray and optical light curves in the rest frame. We use the afterglowpy library to generate synthetic curves to constrain the parameters typical of the bright GRB jet, such as energy (〈E0〉 ∼ 1053.6 erg), opening angle (〈θcore〉 ∼ 0.2 rad), and density (〈n0〉 ∼ 10−2.1 cm−3). Furthermore, we analyse microphysical parameters, including the fraction of thermal energy in accelerated electrons (〈ϵe〉 ∼ 10−1.37) and in the magnetic field (〈ϵB〉 ∼ 10−2.26), and the power-law index of the population of non-thermal electrons (〈p〉 ∼ 2.2).

Calibrating SPI-ACS/INTEGRAL for gamma-ray bursts and re-estimating energetics of GRB/GW 190425 in gamma-ray range

ABSTRACT SPI-ACS/INTEGRAL is one of the most sensitive orbital gamma-ray detectors in energy range above 80 keV. Since 2002 it registered several thousands of gamma-ray bursts (GRBs), including the bursts associated with LIGO-Virgo gravitational wave events GW 170817 and GW 190425. No dedicated in-flight calibrations were performed for SPI-ACS/INTEGRAL, complicating estimation of spectral and energetic characteristics of an event. Using data of GBM/Fermi we perform cross-calibration of SPI-ACS/INTEGRAL, based on 1032 bright GRBs registered by both experiments. We find the conversion factor between instrumental counts from SPI-ACS and energy units from GBM to be dependent on hardness of GRB spectrum (defined as the characteristic energy value, Ep) and on location of a source in spacecraft-based coordinate system. We determine the corresponding analytical model to calculate the conversion factor and estimate its accuracy empirically. Sensitivity of SPI-ACS/INTEGRAL to detect gamma-ray transients is also investigated. Using the calibration we re-estimate energetics of GRB/GW 190425, detected by SPI-ACS/INTEGRAL alone. We constrain possible range of the characteristic energy Ep and isotropic equivalent of total energy, emitted in gamma-rays Eiso for GRB 190425, using the Ep, i–Eiso (Amati) correlation. The calibration model could be applied to any transients with energy spectrum, analogous to GRBs.

Precise measurements of self-absorbed rising reverse shock emission from gamma-ray burst 221009A

The deaths of massive stars are sometimes accompanied by the launch of highly relativistic and collimated jets. If the jet is pointed towards Earth, we observe a ‘prompt’ gamma-ray burst due to internal shocks or magnetic reconnection events within the jet, followed by a long-lived broadband synchrotron afterglow as the jet interacts with the circumburst material. While there is solid observational evidence that emission from multiple shocks contributes to the afterglow signature, detailed studies of the reverse shock, which travels back into the explosion ejecta, are hampered by a lack of early-time observations, particularly in the radio band. We present rapid follow-up radio observations of the exceptionally bright gamma-ray burst GRB 221009A that reveal in detail, both temporally and in frequency space, an optically thick rising component from the reverse shock. From this, we are able to constrain the size, Lorentz factor and internal energy of the outflow while providing accurate predictions for the location of the peak frequency of the reverse shock in the first few hours after the burst. These observations challenge standard gamma-ray burst models describing reverse shock emission.

GRB 221009A afterglow from a shallow angular structured jet

ABSTRACT Exceptionally bright gamma-ray burst (GRB) afterglows can reveal the angular structure of their jets. GRB jets appear to have a narrow core (of half-opening angle θc), beyond which their kinetic energy drops as a power-law with angle θ from the jet’s symmetry axis, $E_{\mathrm{ k},\rm iso}(\theta)\propto [1+(\theta /\theta _\mathrm{ c})^2]^{-a/2}$. The power-law index a reflects the amount of mixing between the shocked jet and confining medium, which depends on the jet’s initial magnetization. Weakly magnetized jets undergo significant mixing, leading to shallow (a ≲ 2) angular profiles. We use the exquisite multiwaveband afterglow observations of GRB 221009A to constrain the jet angular structure using a dynamical model that accounts for both the forward and reverse shocks, for a power-law external density profile, next ∝ R−k. Both the forward shock emission, that dominates the optical and X-ray flux, and the reverse shock emission, that produces the radio afterglow, require a jet with a narrow core (θc ≈ 0.021) and a shallow angular structure (a ≈ 0.8) expanding into a stellar wind (k ≈ 2). Moreover, these data appear to favour a small fraction (ξe ≈ 10−2) of shock heated electrons forming a power-law energy distribution in both shocks.

A tera-electron volt afterglow from a narrow jet in an extremely bright gamma-ray burst.

Some gamma-ray bursts (GRBs) have a tera-electron volt (TeV) afterglow, but the early onset of this has not been observed. We report observations with the Large High Altitude Air Shower Observatory (LHAASO) of the bright GRB 221009A, which serendipitously occurred within the instrument's field of view. More than 64,000 photons >0.2 TeV were detected within the first 3000 seconds. The TeV flux began several minutes after the GRB trigger and then rose to a peak ~10 seconds later. This was followed by a decay phase, which became more rapid ~650 seconds after the peak. We interpret the emission using a model of a relativistic jet with half-opening angle of ~0.8°. This is consistent with the core of a structured jet and could explain the high isotropic energy of this GRB.

GRB 210619B: First Gamma-Ray Burst Detection by the Novel Polarimeter MOPTOP

GRB 210619B was a bright long gamma-ray burst (GRB) which was optically followed up by the novel polarimeter MOPTOP on the Liverpool Telescope. This was the first GRB detection by the instrument since it began science observations. MOPTOP started observing the GRB 1388 s after the Swift Burst Alert Telescope trigger. The R band light-curve decays following a broken power law with a break time of 2948 s after the trigger. The decay index values are α 1 = 0.84 ± 0.03 (pre-break) and α 2 = 0.54 ± 0.02 (post-break), indicating that the observation was most probably during the forward shock-dominated phase. We find a polarization upper limit of ∼7%. In the forward shock we expect the polarization to mostly come from dust in the local ambient medium which only produces low degrees of polarization. Hence our non-detection of polarization is as expected for this particular burst.

Temporal and Spectral Evolution of Gamma-Ray Burst Broad Pulses: Identification of High-latitude Emission in the Prompt Emission

We perform a detailed analysis of broad pulses in bright gamma-ray bursts (GRBs) to understand the evolution of GRB broad pulses. Using the temporal and spectral properties, we test the high-latitude emission (HLE) scenario in the decaying phase of broad pulses. The HLE originates from the curvature effect of a relativistic spherical jet, where higher-latitude photons are delayed and softer than the observer’s line-of-sight emission. The signature of HLE has not yet been identified undisputedly during the prompt emission of GRBs. The HLE theory predicts a specific relation, ∝ , between the peak energy E p in ν F ν spectra and the spectral flux F ν measured at E p , . We search for evidence of this relation in 2157 GRBs detected by the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope from 2008 to 2017. After imposing unbiased selection criteria in order to minimize contamination in a signal by background and overlaps of pulses, we build a sample of 32 broad pulses in 32 GRBs. We perform a time-resolved spectral analysis on each of these 32 broad pulses and find that the evolution of 18 pulses (56%) is clearly consistent with the HLE relation. For the 18 broad pulses, the exponent δ in the relation of ∝ E p δ is distributed as a Gaussian function with a median and width of 1.99 and 0.34, respectively. This result provides a constraint on the emission radius of GRBs with the HLE signature.

GRANDMA and HXMT Observations of GRB 221009A: The Standard Luminosity Afterglow of a Hyperluminous Gamma-Ray Burst—In Gedenken an David Alexander Kann

Object GRB 221009A is the brightest gamma-ray burst (GRB) detected in more than 50 yr of study. In this paper, we present observations in the X-ray and optical domains obtained by the GRANDMA Collaboration and the Insight Collaboration. We study the optical afterglow with empirical fitting using the GRANDMA+HXMT-LE data sets augmented with data from the literature up to 60 days. We then model numerically using a Bayesian approach, and we find that the GRB afterglow, extinguished by a large dust column, is most likely behind a combination of a large Milky Way dust column and moderate low-metallicity dust in the host galaxy. Using the GRANDMA+HXMT-LE+XRT data set, we find that the simplest model, where the observed afterglow is produced by synchrotron radiation at the forward external shock during the deceleration of a top-hat relativistic jet by a uniform medium, fits the multiwavelength observations only moderately well, with a tension between the observed temporal and spectral evolution. This tension is confirmed when using the augmented data set. We find that the consideration of a jet structure (Gaussian or power law), the inclusion of synchrotron self-Compton emission, or the presence of an underlying supernova do not improve the predictions. Placed in the global context of GRB optical afterglows, we find that the afterglow of GRB 221009A is luminous but not extraordinarily so, highlighting that some aspects of this GRB do not deviate from the global known sample despite its extreme energetics and the peculiar afterglow evolution.

Synchrotron Radiation Dominates the Extremely Bright GRB 221009A

The brightest gamma-ray burst, GRB 221009A, has spurred numerous theoretical investigations, with particular attention paid to the origins of ultrahigh-energy TeV photons during the prompt phase. However, analyzing the mechanism of radiation of photons in the ∼MeV range has been difficult because the high flux causes pileup and saturation effects in most GRB detectors. In this Letter, we present systematic modeling of the time-resolved spectra of the GRB using unsaturated data obtained from the Fermi Gamma-ray Burst Monitor (precursor) and SATech-01/GECAM-C (main emission and flare). Our approach incorporates the synchrotron radiation model, which assumes an expanding emission region with relativistic speed and a global magnetic field that decays with radius, and successfully fits such a model to the observational data. Our results indicate that the spectra of the burst are fully in accordance with a synchrotron origin from relativistic electrons accelerated at a large emission radius. The lack of thermal emission in the prompt emission spectra supports a Poynting flux–dominated jet composition.

Two-component jet model for multiwavelength afterglow emission of the extremely energetic burst GRB 221009A

ABSTRACT Recently gamma-ray bursts (GRBs) have been detected at very-high-energy (VHE) gamma-rays by imaging atmospheric Cherenkov telescopes, and a two-component jet model has often been invoked to explain multiwavelength data. In this work, multiwavelength afterglow emission from an extremely bright GRB, GRB 221009A, is examined. The isotropic-equivalent gamma-ray energy of this event is among the largest, which suggests that similarly to previous VHE GRBs, the jet opening angle is so small that the collimation-corrected gamma-ray energy is nominal. Afterglow emission from such a narrow jet decays too rapidly, especially if the jet propagates into uniform circumburst material. In the two-component jet model, another wide jet component with a smaller Lorentz factor dominates late-time afterglow emission, and we show that multiwavelength data of GRB 221009A can be explained by narrow and wide jets with opening angles similar to those employed for other VHE GRBs. We also discuss how model degeneracies can be disentangled with observations.

The IXPE View of GRB 221009A

We present the IXPE observation of GRB 221009A, which includes upper limits on the linear polarization degree of both prompt and afterglow emission in the soft X-ray energy band. GRB 221009A is an exceptionally bright gamma-ray burst (GRB) that reached Earth on 2022 October 9 after traveling through the dust of the Milky Way. The Imaging X-ray Polarimetry Explorer (IXPE) pointed at GRB 221009A on October 11 to observe, for the first time, the 2–8 keV X-ray polarization of a GRB afterglow. We set an upper limit to the polarization degree of the afterglow emission of 13.8% at a 99% confidence level. This result provides constraints on the jet opening angle and the viewing angle of the GRB, or alternatively, other properties of the emission region. Additionally, IXPE captured halo-rings of dust-scattered photons that are echoes of the GRB prompt emission. The 99% confidence level upper limit to the prompt polarization degree depends on the background model assumption, and it ranges between ∼55% and ∼82%. This single IXPE pointing provides both the first assessment of X-ray polarization of a GRB afterglow and the first GRB study with polarization observations of both the prompt and afterglow phases.

The Power of the Rings: The GRB 221009A Soft X-Ray Emission from Its Dust-scattering Halo

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected that has occurred at low Galactic latitude. Owing to this exceptional combination, its prompt X-ray emission could be detected for weeks in the form of expanding X-ray rings produced by scattering in Galactic dust clouds. We report on the analysis of 20 rings, generated by dust at distances ranging from 0.3 to 18.6 kpc, detected during two X‐ray Multi Mirror (XMM)-Newton observations performed about 2 and 5 days after the GRB. By fitting the spectra of the rings with different models for the dust composition and grain size distribution, we reconstructed the spectrum of the GRB prompt emission in the 0.7–4 keV energy range as an absorbed power law with photon index Γ = 1–1.4 and absorption in the host galaxy N H,z = (4.1–5.3) × 1021 cm−2. Taking into account the systematic uncertainties regarding the column density of dust contained in the clouds producing the rings, the 0.5–5 keV fluence of GRB 221009A can be constrained between 10−3 and 7 × 10−3 erg cm−2. Both the fluence and the photon index indicate the presence of a possible soft excess with respect to the extrapolation of the main GRB peak observed at higher energies.

GRB 221009A: The BOAT

GRB 221009A has been referred to as the brightest of all time (BOAT). We investigate the veracity of this statement by comparing it with a half century of prompt gamma-ray burst observations. This burst is the brightest ever detected by the measures of peak flux and fluence. Unexpectedly, GRB 221009A has the highest isotropic-equivalent total energy ever identified, while the peak luminosity is at the ∼99th percentile of the known distribution. We explore how such a burst can be powered and discuss potential implications for ultralong and high-redshift gamma-ray bursts. By geometric extrapolation of the total fluence and peak flux distributions, GRB 221009A appears to be a once-in-10,000-year event. Thus, it is almost certainly not the BOAT over all of cosmic history; it may be the brightest gamma-ray burst since human civilization began.

H.E.S.S. Follow-up Observations of GRB 221009A

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE; >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of above E thr = 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.