Models Of Gamma-ray Bursts Research Articles

Evidence for rapid variability at high energies in GRBs

ABSTRACT Intrinsic variability was searched for in arrival times of six gamma-ray bursts (GRBs) at high energies – between 30 MeV and 2 GeV – detected by the Fermi satellite’s Large Area Telescope (LAT). The GRBs were selected from the Fermi LAT catalogue with preference for events with numerous photons, a strong initial pulse, and measured redshifts. Three long GRBs and three short GRBs were selected and tested. Two different variability-detection algorithms were deployed, one counting photons in pairs, and the other multiplying time gaps between photons. In both tests, a real GRB was compared to 1000 Monte Carlo versions of itself smoothed over a wide range of different time-scales. The minimum detected variability time-scales for long bursts (GRB 080916C, GRB 090926A, GRB 131108A) was found to be (0.005, 10.0, 10.0) s for the photon pair test and (2.0, 20.0, 10.0) s for the time-gap multiplication test. Additionally, the minimum detected variability time-scales for the short bursts (GRB 090510, GRB 140619B, GRB 160709A) was found to be (0.05, 0.01, 20.0) s for the photon pair test and (0.05, 0.01, 20.0) s for the gap multiplication test. Statistical uncertainties in these times are about a factor of 2. The durations of these variability time-scales may be used to constrain the geometry, dynamics, speed, cosmological dispersion, Lorentz-invariance violations, weak equivalence principle violations, and GRB models.

Cascade Radiations of e ± from γγ-annihilation Process as an Extra Component of the Early Optical/X-Ray Afterglows of Gamma-Ray Bursts

Chromatic break and/or plateau observed in the early optical and X-ray afterglow lightcurves challenge the conventional external shock models of gamma-ray bursts (GRBs). Detection of TeV gamma-ray afterglows indicates strong gamma-ray production within the afterglow jets. We investigate the cascade radiations of the e ± production via the γ γ interaction in the jets. Our numerical calculations show that the cascade synchrotron emission can make a significant contribution to the early optical/X-ray afterglows. The combination of the primary and cascade emission fluxes can shape a chromatic break and/or plateau in the early optical/X-ray lightcurves, depending on the jet properties. Applying our model to GRBs 050801 and 080310, we found that their optical plateaus and the late X-ray/optical lightcurves can be explained with our model in reasonable parameter values. We suggest that such a chromatic optical plateau could be a signature of strong e ± production in GRB afterglow jets. The TeV gamma-ray flux of such GRBs should be significantly reduced and hence tends to be detectable for those GRBs that have a single power-law decaying optical afterglow lightcurve.

Ending the prompt phase in photospheric models of gamma-ray bursts

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all gamma-ray burst central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can give us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.

Evidence of High-latitude Emission in the Prompt Phase of GRBs: How Far from the Central Engine are the GRBs Produced?

One of the difficulties in nailing down the physical mechanism of gamma-ray bursts (GRBs) comes from the fact that there has been no clear observational evidence on how far from the central engine the prompt gamma rays of GRBs are emitted. Here we present a simple study addressing this question by making use of the “high-latitude emission” (HLE). We show that our detailed numerical modeling exhibits a clear signature of HLE in the decaying phase of “broad pulses” of GRBs. We show that the HLE can emerge as a prominent spectral break in F ν spectra and dominate the peak of ν F ν spectra even while the “line-of-sight emission” (LoSE) is still ongoing. This finding provides a new view of HLE emergence since it has been believed so far that the HLE can show up and dominate the spectra only after the LoSE is turned off. We remark, however, that this “HLE break” can be hidden in some broad pulses, depending on the proximity between the peak energies of the LoSE and the HLE. Therefore, this new picture of HLE emergence explains both the detection and nondetection of HLE signature in observations of broad pulses. Also, we present three examples of Fermi Gamma-ray Burst Monitor GRBs with broad pulses that exhibit the HLE signature. We show that their gamma-ray-emitting region should be located at ∼1016 cm from the central engine, which places a constraint on the GRB models.

Parameter Inference of a State-of-the-Art Physical Afterglow Model for GRB 190114C

A state-of-the-art semi-analytic gamma-ray burst (GRB) afterglow model with synchrotron self-Compton (SSC) emission has been applied for the first time for parameter inference using real GRB data. We analyzed the famous GRB 190114C as a case study. GRB 190114C, characterized by its long duration and high luminosity, was observed by many ground-based and orbiting telescopes spanning a wide range of electromagnetic wavelengths, from radio to GeV gamma rays. We used two advanced algorithms for inference: a nested sampling algorithm called UltraNest and an MCMC algorithm emcee. Evoking the standard afterglow model, the inference result and the best-fit values lead to an initial bulk Lorentz factor (a rough estimate of Γ=526), which aligns with the values often seen in GRBs identified by the Fermi-LAT instrument. Similarly to the best-fit values of other studies in the literature, the derived values of microphysical parameters, the circumburst density, and the kinetic efficiency are consistent with those found after modeling the multi-wavelength observations in GRB 190114C. We show that the SSC from the forward-shock region can only describe the highest-energy photons above a few GeVs.

Reverse shock emission in an off-axis top-hat jet model for gamma-ray bursts

ABSTRACT The afterglow of a gamma-ray burst (GRB) has been widely argued to arise from the interaction of a relativistic outflow with its ambient medium. During such an interaction, a pair of shocks are generated: a forward shock that propagates into the medium and a reverse shock that propagates into the outflow. Extensive studies have been conducted on the emission from the forward shock viewed off-axis. Furthermore, the observation of a reverse shock in an on-axis short GRB suggests that the reverse shock can produce an electromagnetic counterpart to a gravitational wave-detected merger. In this paper, we investigate the contribution of the reverse shock to the afterglow from a top-hat jet viewed off-axis, and apply our model to some short GRBs previously modelled by an off-axis emission. We employ the Markov Chain Monte Carlo (MCMC) method to get the model parameters (i.e. the jet’s half-opeaning angle θj, the viewing angle θobs, the initial Lorentz factor Γ0, and the isotropic energy Eiso). Our model successfully reproduces off-axis afterglow emission without a structured jet. In addition, our calculations suggest that the reverse shock may produce a prominent feature in an early afterglow, which can be potentially observed in an orphan optical afterglow.

The short gamma-ray burst population in a quasi-universal jet scenario

We present a model of the short gamma-ray burst (SGRB) population under a ‘quasi-universal jet’ scenario in which jets can differ somewhat in their on-axis peak prompt emission luminosity, Lc, but share a universal angular luminosity profile, ℓ(θv) = L(θv)/Lc, as a function of the viewing angle, θv. The model was fitted, through a Bayesian hierarchical approach inspired by gravitational wave (GW) population analyses, to three observed SGRB samples simultaneously: the Fermi/GBM sample of SGRBs with spectral information available in the catalogue (367 events); a flux-complete sample of 16 Swift/BAT SGRBs that are also detected by the GBM and have a measured redshift; and a sample of SGRBs with a binary neutron star (BNS) merger counterpart, which only includes GRB 170817A at present. Particular care was put into modelling selection effects. The resulting model, which reproduces the observations, favours a narrow jet ‘core’ with half-opening angle θc = 2.1−1.4+2.4 deg (uncertainties hereon refer to 90% credible intervals from our fiducial ‘full sample’ analysis) whose peak luminosity, as seen on-axis, is distributed as a power law, p(Lc) ∝ Lc−A with A = 3.2−0.4+0.7, above a minimum isotropic-equivalent luminosity, Lc⋆ = 5−2+11 × 1051 erg s−1. For viewing angles larger than θc, the luminosity profile scales as a single power law, l ∝ θv−αL with αL = 4.7−1.4+1.2, with no evidence of a break, despite the model allowing for it. While the model implies an intrinsic ‘Yonetoku’ correlation between L and the peak photon energy, Ep, of the spectral energy distribution, its slope is somewhat shallower, Ep ∝ L0.4 ± 0.2, than the apparent one, and the normalisation is offset towards larger Ep due to selection effects. The implied local rate density of SGRBs (regardless of the viewing angle) is between about one hundred up to several thousand events per cubic gigaparsec per year, in line with the BNS merger rate density inferred from GW observations. Based on the model, we predict 0.2 to 1.3 joint GW+SGRB detections per year by the advanced GW detector network and Fermi/GBM during the O4 observing run.

Evolution of fallback discs around millisecond magnetars: effect of supercritical accretion on GRB afterglows

ABSTRACT Some models of gamma-ray bursts (GRBs) invoke nascent millisecond magnetars as the central engine and address the X-ray afterglows with the interaction of magnetar magnetospheres with fallback discs. We study the evolution of fallback discs interacting with the millisecond magnetars. Initially, the accretion rate in the fallback disc is very high, well above the rate required for the Eddington limit. The inner parts of such a disc, even if it is cooling by the neutrino emission, get spherical due to the radiation pressure, which regulates the mass-accretion rate within the spherization radius. Such a disc can not penetrate the light cylinder radius for the typical magnetic fields, and the initial spin frequencies invoked for the magnetars. As a result of the autoregulation of the accretion flow, the fallback disc can not interact directly with the magnetar’s magnetosphere within the first few days. This has implications for the fallback disc models of GRB afterglows since the accretion and propeller luminosities, in the presence of radiation pressure, are too low to address the typical luminosities of X-ray afterglows.

Accretion spin-up of the massive component in the neutron star stripping model for short gamma-ray bursts

ABSTRACT In this paper, we use analytical methods to study the last stages of the double neutron star (NS) system evolution. Depending on the initial masses of the components, this evolution can occur either in the framework of the merging scenario or in the NS stripping model. The main new ingredient of this work, compared with previous calculations, is accounting for accretion spin-up of the massive component. This effect leads to a significant decrease in the duration of the stable mass transfer of matter in the stripping mechanism. Within the framework of the Newtonian approximation, we determine the boundary between the merging and stripping scenarios. It is shown that this boundary weakly depends on the total mass of the system and the specific form of the NS equation of state, and is determined mainly by the initial mass ratio of the components. The stripping scenario is realized at M2/M1 ≲ 0.8, so it should make a large contribution to the population of gravitational wave events from NS–NS coalescing binaries that are close to us, and their accompanying short gamma-ray bursts. Nevertheless, the value obtained requires further clarification, taking into account relativistic effects, possible non-conservative mass transfer, etc.

Stripping Model for Short GRBs: The Impact of Nuclear Data

We investigate the impact of forthcoming nuclear data on the predictions of the neutron star (NS) stripping model for short gamma-ray bursts. The main area to which we pay attention is the NS crust. We show that the uncertain properties of the NS equation of state can significantly influence the stripping time tstr, the main dynamical parameter of the model. Based on the known time delay (tstr≈1.7 s) between the peak of the gravitational wave signal GW170817 and the detection of gamma photons from GRB170817A, we obtain new restrictions on the nuclear matter parameters, in particular, the symmetry energy slope parameter: L<114.5MeV. In addition, we study the process of nucleosynthesis in the outer and inner crusts of a low-mass NS. We show that the nucleosynthesis is strongly influenced by both the forthcoming nuclear data and the equation of state of the NS matter.

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.

The role of a standard family of Ic supernovae in BDHN I, BDHN II, and BDHN III GRBs

AbstractA profound difference has occurred in the analysis of Gamma Ray Bursts (GRBs) initially analyzed in the domain of gamma‐ray astronomy with the Compton Gamma Ray Observatory and the BATSE Detector with the extension to x‐ray astronomy and optical astronomy introduced by Beppo‐SAX and the KEK and VLT optical observatory, followed by the AGILE, Fermi mission, Niels Gehrels SWIFT mission as well as HEHSS and MAGIC observations An authentic “Copernican revolution” has occurred in the transition from the traditional model of GRBs originating in a single star progenitor (collapsar) to GRBs with a binary progenitor composed of a CO core and a companion NS. We have evidence from 24 SN observations related to GRBs that all of them, once analyzed with the general relativistic corrections can be identified with I Bc Sn with a common value of the Luminosity and common time of occurrence of the peak of the optical emission. This can be understood in terms of a precursor composed of a CO core and a binary NS companion. By contrast, the GRBs differ profoundly in their energetics which can be expressed in terms of 3 different classes of BDHNe: BDHN I, BDHN II, and BDHN III, also originating from Precursors composed of a CO core and a Binary companion. It is pointed out how the analysis of these systems has profoundly modified the concept of the BH and the associated fundamental physics necessary for their description since the “introducing of the Black Hole” by Ruffini and Wheeler in 1971. At first, it is illustrated how the most radical change has occurred in the introduction of the effective BH charge, with an electric field only function of the BH dimensionless spin and a background magnetic field Bo. This definitely indicates the abandonment of the Kerr‐Newman solution in favor of the Kerr metric, for energy extraction processes in BH physics. We then enter into the detailed description of the “seven Seals,” the seven Episodes characterizing GRB 190114c, and the associated SN 2019jrj.This is based on the article by Aimuratov et al. (in preparation).

DeepGlow: An efficient neural network emulator of physical afterglow models for gamma-ray bursts and gravitational-wave events

Abstract Gamma-ray bursts (GRBs) and double neutron star merger gravitational-wave events are followed by afterglows that shine from X-rays to radio, and these broadband transients are generally interpreted using analytical models. Such models are relatively fast to execute, and thus easily allow estimates of the energy and geometry parameters of the blast wave, through many trial-and-error model calculations. One problem, however, is that such analytical models do not capture the underlying physical processes as well as more realistic relativistic numerical hydrodynamic (RHD) simulations do. Ideally, those simulations are used for parameter estimation instead, but their computational cost makes this intractable. To this end, we present DeepGlow, a highly efficient neural network architecture trained to emulate a computationally costly RHD-based model of GRB afterglows, to within a few percent accuracy. As a first scientific application, we compare both the emulator and a different analytical model calibrated to RHD simulations, to estimate the parameters of a broadband GRB afterglow. We find consistent results between these two models, and also give further evidence for a stellar wind progenitor environment around this GRB source. DeepGlow fuses simulations that are otherwise too complex to execute over all parameters, to real broadband data of current and future GRB afterglows.

Constraints on Gamma-Ray Burst Models from GRB 221009A: GeV Gamma Rays versus High-energy Neutrinos

Gamma-ray bursts (GRBs) are generally believed to be efficient particle accelerators. In the presence of energetic protons in a GRB jet, interactions between these protons and the intense radiation field of the GRB are supposed to induce an electromagnetic cascade. Electrons/positrons generated in the cascade will produce an additional spectrum of a robust feature, which is in the form of a power-law distribution up to a GeV regime with an index of ≲2. We suggest that measurements of the Fermi Large Area Telescope at the GeV band can provide independent constraints on the key GRB model parameters such as the dissipation radius, the jet’s bulk Lorentz factor, and the baryon loading factor. Taking GRB 221009A, the brightest GRB ever detected, as an example, we show that the constraints from GeV gamma-ray emission may be more stringent than that from the neutrino observation, providing us with deep insight into the origin of GRBs.

Synchrotron self-compton emission in the two-component jet model for gamma-ray bursts

Gamma-ray bursts (GRBs) are intense bursts of high-energy photons (prompt emissions) caused by relativistic jets. After the emissions, multi-wavelength afterglows, from radio to very-high-energy (VHE) gamma-ray, last for more than a few days. In the past three years, the VHE gamma-ray photons from four GRBs (GRBs 180720B, 190114C, 190829A and 201216C) were detected by ground-based Imaging Atmospheric Cherenkov Telescopes, such as the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.). One of them, GRB 190829A, had some peculiar features of showing achromatic peaks in X-ray and optical bands at 1.4×103 s and being classified as low-luminosity GRBs. Previously, we proposed a two-component jet model, which has ‘narrow jet’ with a small initial opening half-angle θ0=0.015 rad and large bulk Lorentz factor Γ0=350, and ‘wide jet’ with θ0=0.1 rad and Γ0=20. The narrow jet explained the early X-ray and optical emissions and apparently small isotropic gamma-ray energy and peak energy in the off-axis viewing case. Furthermore, the late X-ray and radio (1.3 and 15.5 GHz) afterglows were emitted from the wide jet. Here, we calculate the VHE gamma-ray flux by the synchrotron self-Compton (SSC) emission. The multi-wavelength afterglows of GRB 190829A including the VHE gamma-ray emission are well explained by our two-component jet model. The afterglow emissions from our two-component jet are also consistent with the observational results of GRBs 180720B, 190114C and 201216C, when the jets are viewed on-axis. Furthermore, we discuss the detectability of off-axis orphan afterglows by the Cherenkov Telescope Array (CTA).

Neutrinos from the Brightest Gamma-Ray Burst?

We discuss implications that can be obtained by searches for neutrinos from the brightest gamma-ray burst (GRB), GRB 221009A. We derive constraints on GRB model parameters such as the cosmic-ray loading factor and dissipation radius, taking into account both neutrino spectra and effective areas. The results are strong enough to constrain proton acceleration near the photosphere, and we find that the single burst limits are comparable to those from stacking analysis. Quasi-thermal neutrinos from subphotospheres and ultra-high-energy neutrinos from external shocks are not yet constrained. We show that GeV–TeV neutrinos originating from neutron collisions are detectable, and urge dedicated analysis on these neutrinos with DeepCore and IceCube as well as ORCA and KM3NeT.

The Maximum Isotropic Equivalent Energy of Gamma-Ray Bursts

In the cannonball model of gamma-ray bursts (GRBs), a highly relativistic jet of plasmoids of ordinary stellar matter that is ejected during stellar collapse or shortly after by fallback matter, produces simultaneously a GRB and a cosmic-ray burst by scattering light and charged particles in its path. This association and the observed knee at ∼1 TeV in the energy spectrum of Galactic cosmic-ray electrons imply a maximum peak energy ∼2.25 MeV in the energy spectrum of GRBs in the 1 keV–10 MeV band. Such a peak energy and the Amati correlation in GRBs imply a maximum isotropic equivalent energy release of ∼3.8 × 1054 erg in GRBs, in the 1 keV–10 MeV band. Both predictions are in good agreement with up-to-date observations.

Modeling the Gamma-Ray Burst Jet Properties with 3D General Relativistic Simulations of Magnetically Arrested Accretion Flows

We investigate the dependence of the gamma-ray burst (GRB) jet structure and its evolution on the properties of the accreting torus in the central engine. Our models numerically evolve the accretion disk around a Kerr black hole using three-dimensional general relativistic magnetohydrodynamic simulations. We use two different analytical hydrodynamical models of the accretion disk, based on the Fishbone–Moncrief and Chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star (BNS) merger, respectively. We impose poloidal magnetic fields of two different geometries upon the initial stable solutions. We study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. The jets produced in our models are structured and have a relatively hollow core and reach higher Lorentz factors at an angle ≳9° from the axis. The jet in our short GRB model has an opening angle of up to ∼25° while our long GRB engine produces a narrower jet, of up to ∼11°. We also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. The application of our models to the GRB jets in the BNS postmerger system and to the ultrarelativistic jets launched from collapsing stars are briefly discussed.

Critical Tests of Leading Gamma Ray Burst Theories

It has been observationally established that supernovae (SNe) of Type Ic produce long duration gamma-ray bursts (GRBs) and that neutron star mergers generate short hard GRBs. SN-Less GRBs presumably originate in a phase transition of a neutron star in a high mass X-ray binary. How these phenomena actually generate GRBs is debated. The fireball and cannonball models of GRBs and their afterglows have been widely confronted with the huge observational data, with their defenders claiming success. The claims, however, may reflect multiple choices and the use of many adjustable parameters, rather than the validity of the models. Only a confrontation of key falsifiable predictions of the models with solid observational data can test their validity. Such critical tests are reviewed in this report.

Stripping Model for Short Gamma-Ray Bursts in Neutron Star Mergers

We overview the current status of the stripping model for short gamma-ray bursts. After the historical joint detection of the gravitational wave event GW170817 and the accompanying gamma-ray burst GRB170817A, the relation between short gamma-ray bursts and neutron star mergers has been reliably confirmed. Many properties of GRB170817A, which turned out to be peculiar in comparison with other short gamma-ray bursts, are naturally explained in the stripping model, suggested by one of us in 1984. We point out the role of late Dmitriy Nadyozhin (1937–2020) in predicting the GRB and kilonova properties in 1990. We also review the problems to be solved in the context of this model.