GRB 080319B Research Articles

Possible X-ray cocoon emission from GRB 050709

Abstract The detection of the short gamma-ray burst (SGRB) 050709 by the HETE-2 satellite opened the door to understanding the nature of SGRBs, offering clues about their emission mechanism and progenitors, with the crucial aid of optical follow-up observations. Here, we revisit the prompt emission of GRB 050709. Our analysis reveals an initial hard spike ∼200 ms long, followed by a subsequent soft-tail emission lasting ∼300 ms. These components could be common among other SGRBs originating from binary neutron merger events, such as GW 170817 and GRB 170817A. Detailed temporal and spectral analyses indicate that the soft-tail emission might be attributed to the cocoon formed by the relativistic jet depositing energy into the surrounding material. We find the necessary cocoon parameters at the breakout, as consistent with numerical simulation results. We compared the physical parameters of this cocoon with those of other SGRBs. The relatively higher cocoon pressure and temperature in GRB 050709 may indicate a more on-axis jet compared with GRB 170817A and GRB 150101B.

Very high energy gamma-rays from GRB 180720B and GRB 190829A with external Compton emission

ABSTRACT Gamma-ray bursts (GRBs) comprise short, bright, energetic flashes of emission from extragalactic sources followed by a longer afterglow phase of decreased brightness. Recent discoveries of very high energy (VHE, ≳100 GeV) afterglow emission from GRB 180720B and GRB 190829A by the High Energy Stereoscopic System have raised questions regarding the emission mechanism responsible. We interpret this observed late-time emission to be the result of inverse Compton emission of ultrarelativistic electrons in the GRB blast wave in an external radiation field, i.e. external Compton (EC), considering both the wind and interstellar medium scenarios. We present predictions of multiwavelength light curves and energy spectra, ranging from optical to VHE, and include the synchrotron and synchrotron self-Compton (SSC) radiation mechanisms as well. We corrected the EC and SSC models for the gamma-ray attenuation by absorption of photons through their interaction with the extragalactic background light. We compared our results to multiwavelength data and found that EC gives a satisfactory fit for a given set of fixed model parameters for GRB 180720B, whereas SSC results in a better fit for GRB 190829A. For both GRBs, a wind environment is preferred over constant-density interstellar medium, and the cosmic microwave background as the external radiation field. However, with more data and an effective optimization tool we can find a more robust fit of the model, implying better constraints on the GRB environment and the particle energy requirements for the emission observed at late times. This has consequences for future observations of GRBs at these extreme energies.

Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B

Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B

Microphysical parameter variation in gamma-ray burst stratified afterglows and closure relations: from sub-GeV to TeV observations

ABSTRACT Gamma-ray bursts (GRBs) are one of the most exciting sources that offer valuable opportunities for investigating the evolution of energy fraction given to magnetic fields and particles through microphysical parameters during relativistic shocks. The delayed onset of GeV–TeV radiation from bursts detected by the Fermi Large Area Telescope and Cherenkov Telescopes provide crucial information in favour of the external-shock model. Derivation of the closure relations (CRs) and the light curves in external shocks requires knowledge of GRB afterglow physics. In this manuscript, we derive the CRs and light curves in a stratified medium with variations of microphysical parameters of the synchrotron and synchrotron self-Compton (SSC) afterglow model radiated by an electron distribution with a hard and soft spectral index. Using Markov chain Monte Carlo simulations, we apply the current model to investigate the evolution of the spectral and temporal indices of those GRBs reported in the Second Gamma-ray Burst Catalog (2FLGC), which comprises 29 bursts with photon energies above 10 GeV and of those bursts (GRB 180720B, 190114C, 190829A, and 221009A) with energetic photons above 100 GeV, which can hardly be modelled with the CRs of the standard synchrotron scenario. The analysis shows that (i) the most likely afterglow model using synchrotron and SSC emission on the 2FLGC corresponds to the constant-density scenario, and (ii) variations of spectral (temporal) index keeping the temporal (spectral) index constant could be associated with the evolution of microphysical parameters, as exhibited in GRB 190829A and GRB 221009A.

On the nature of the ultrarelativistic prompt emission phase of GRB 190114C and GRB 180720B

A profound difference has occurred in the analysis of 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 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, 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 publication of ``Introducing 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 $B0$. This definitely indicates the abandonment of the Kerr--Newman solution in favour of the Kerr metric, for energy extraction processes in BH physics. We then enter in the detailed description of the ``seven Seals", the seven Episodes characterizing GRB 190114c and the associated SN 2019jrj. Based on the article by Aimuratov et al., in preparation.

The structure of the ultrarelativistic prompt emission phase and the properties of the black hole in GRB 180720B

In analogy with GRB 190114C, we here analyze the ultrarelativistic prompt emission (UPE) of GRB 180720B observed in the rest-frame time interval t_{mathrm{rf}}=4.84–10.89 s by Fermi-GBM. We reveal the UPE hierarchical structure from the time-resolved spectral analysis performed in time sub-intervals: the spectrum in each shorter time interval is always fitted by a composite blackbody plus cutoff power-law model. We explain this structure with the inner engine of binary-driven hypernova (BdHN) model operating in a quantum electrodynamics (QED) regime. In this regime, the electric field induced by the gravitomagnetic interaction of the newborn Kerr BH with the surrounding magnetic field is overcritical, i.e., |mathbf{E}|ge E_c, where E_c=m_e^2 c^3/(ehbar ). The overcritical field polarizes the vacuum leading to an e^+~e^- pair plasma that loads baryons from the surroundings during its expansion. We calculate the dynamics of the self-acceleration of the pair-electromagnetic-baryon (PEMB) pulses to their point of transparency. We characterize the quantum vacuum polarization process in the sequences of decreasing time bins of the UPE by determining the radiation timescale, Lorentz factors, and transparency radius of the PEMB pulses. We also estimate the strength of the surrounding magnetic field sim 10^{14} G, and obtain a lower limit to the BH mass, M=2.4~M_odot , and correspondingly an upper limit to the spin, alpha = 0.6, from the conditions that the UPE is powered by the Kerr BH extractable energy and its mass is bound from below by the NS critical mass.

Very High-energy Afterglow Emission of GRB 190829A: Evidence for Its Hadronic Origin?

The detection of multi-TeV gamma rays from the afterglow phase of GRB 190829A by the High Energy Stereoscopic System telescope is an addition to the already existing list of two GRBs observed in very high-energy (VHE) gamma rays in recent years. Jets of blazars and GRBs have many similarities and the photohadronic model is very successful in explaining the VHE gamma-ray spectra from the high-energy blazars. Recently, the photohadronic model has been successfully applied to study the sub-TeV gamma rays from the afterglow phases of GRB 180720B and GRB 190114C. We employed this model again to explain the VHE spectra observed for the two consecutive nights from GRB 190829A. We show that the spectra of GRB 190829A can be due to the interactions of high-energy protons with the synchrotron self-Compton photons in the forward shock region of the GRB jet, similar to the low emission state of the VHE flaring events of high-energy blazars. We speculate that, if in the future, it is possible to observe the VHE gamma-ray spectra from nearby GRBs in their afterglow phases, then some of them could only be explained by employing two different spectral indices. If confirmed, such VHE spectra could be interpreted as a result of the interactions of the high-energy protons with the photons, both from the synchrotron background and the synchrotron self-Compton background in the forward shock region.

OBSERVATIONAL PROPERTIES OF TEV DETECTED GRB 180720B, GRB 190114C AND GRB 190829A

TeV emissions from γ-ray bursts are very important to study their origin and the radiation mechanisms in detail. Recent observations of TeV photons in some of the GRBs are challenging to be explained by the traditional Synchrotron radiation mechanism. In this work, we present the results of a detailed investigation of the prompt and afterglow emissions of recently discovered TeV GRBs (GRB 180720B, GRB 190114C, and GRB 190829A) based on the publicly available prompt and afterglow data including 10.4m GTC and 1.3m DFOT telescopes observations of the first HESS and MAGIC bursts, respectively. Timeresolved spectroscopy of prompt emission of GRB 180720B and GRB 190114C shows an intensity tracking nature of peak energy. In the case of GRB 190829A, peak energy evolution shows a hard to soft tracking trend followed by a very soft and chaotic trend. GRB 190829A is a peculiar intermediate luminous two episodic burst with first emission episode outlier to Amati correlation. We analyzed the late time Fermi-LAT emission that encapsulates the H.E.S.S. and MAGIC observations. Some of the LAT photons are likely to be associated with these GRBs and they could have an Inverse Compton radiation mechanism.

Infrared dust echoes from neutron star mergers

ABSTRACT A significant fraction of binary neutron star mergers occur in star-forming galaxies where the UV-optical and soft X-ray emission from the relativistic jet may be absorbed by dust and re-emitted at longer wavelengths. We show that, for mergers occurring in gas-rich environment ($n_{\rm H}\gtrsim 0.5\rm \, cm^{-3}$ at a few to tens of pc) and when the viewing angle is less than about 30°, the emission from heated dust should be detectable by James Webb Space Telescope (JWST), with a detection rate of $\sim 1\rm \, yr^{-1}$. The spatial separation between the dust emission and the merger site is a few to 10 milli-arcsecs (for a source distance of 150 Mpc), which may be astrometrically resolved by JWST for sufficiently high signal-noise-ratio detections. Measuring the superluminal apparent speed of the flux centroid directly gives the orbital inclination of the merger, which can be combined with gravitational wave data to measure the Hubble constant. For a line of sight within the jet opening angle, the dust echoes are much brighter and may contaminate the search for kilonova candidates from short gamma-ray bursts, such as the case of GRB 130603B.

Jitter radiation: towards TeV-photons of gamma-ray bursts

ABSTRACT The synchrotron mechanism has the radiation limit of about 160 MeV, and it is not possible to explain the very high energy (VHE) photons that are emitted by high-energy objects. Inverse Compton scattering as a traditional process is applied for the explanation of the VHE emission. In this paper, jitter radiation, the relativistic electron radiation in the random and small-scale magnetic field, is proposed to be a possible mechanism to produce VHE photons. The jitter radiation frequency is associated with the perturbation field. The spectral index of the jitter radiation is dominated by the kinetic turbulence. We utilize the jitter radiation to explain the gamma-ray burst (GRB 190114C and GRB 180720B) VHE emissions that were recently detected by the Imaging Atmospheric Cherenkov Telescopes. We suggest that this mechanism can be applied to other kinds of VHE sources.

ANTARES upper limits on the multi-TeV neutrino emission from the GRBs detected by IACTs

The first gamma-ray burst detections by Imaging Atmospheric Cherenkov Telescopes have been recently announced: GRB 190114C, detected by MAGIC, GRB 180720B and GRB 190829A, observed by H.E.S.S. A dedicated search for neutrinos in space and time coincidence with the gamma-ray emission observed by IACTs has been performed using ANTARES data. The search covers both the prompt and afterglow phases, yielding no neutrinos in coincidence with the three GRBs studied. Upper limits on the energetics of the neutrino emission are inferred. The resulting upper limits are several orders of magnitude above the observed gamma-ray emission, and they do not allow to constrain the available models.

Looking for the possible gluon condensation signature in sub-TeV gamma-ray spectra: from active galactic nuclei to gamma ray bursts

The gluon condensation in the proton as a dynamical model is used to treat a series of unsolved puzzles in sub-TeV gamma ray spectra, they include the broken power-law of blazar's radiation, the hardening confusion of 1ES 1426+428, Mkn 501, and the recently recorded sub-TeV gamma spectra of GRB 180720B and GRB 190114C . We find that the above anomalous phenomena in gamma ray energy spectra can be understood with the simple broken power law based on a QCD gluon condensation effect.

Origin of Sub-TeV Afterglow Emission from Gamma-Ray Bursts GRB 190114C and GRB 180720B

Abstract The detection of GRB 180722B and GRB 190114C in sub-TeV gamma-rays has opened up a new window to study gamma-ray bursts in high-energy gamma-rays. Recently it has been shown that the synchrotron and inverse Compton processes are responsible for the production of these high-energy gamma-rays during the afterglow. Here, for the first time we demonstrate that the photohadronic scenario that is successful in explaining the multi-TeV flaring in high-energy blazars is also applicable for gamma-ray bursts. We show that the sub-TeV spectra of GRB 190114C and GRB 180720B are due to the interaction of high-energy protons with the background photons in the synchrotron self-Compton region and synchrotron region, respectively. The nature of the background photon distributions help us to constrain their bulk Lorentz factors.

Description of Atypical Bursts Seen Slightly Off-axis

Abstract The detection of gravitational waves together with their electromagnetic counterpart, in the gamma-ray burst GRB 170817A, marked a new era of multi-messenger astronomy. Several theoretical models have been proposed to explain the atypical behavior of this event. Recently, it was shown th0at the multiwavelength afterglow of GRB 170817A was consistent with a synchrotron forward-shock model when the outflow was viewed off-axis, decelerated in a uniform medium and parameterized through a power-law velocity distribution. Motivated by the upper limits on the very high-energy emission, and the stratified medium in the close vicinity of a binary neutron star merger proposed to explain the gamma-ray flux in the short GRB 150101B, we extend the mechanism proposed to explain GRB 170817A to a more general scenario deriving the synchrotron self-Compton and synchrotron forward-shock model when the off-axis outflow is decelerated in a uniform and stratified circumburst density. As particular cases, we show that the delayed and long-lasting afterglow emission observed in GRB 080503, GRB 140903A, GRB 150101B, and GRB 160821B could be interpreted by a scenario similar to the one used to describe GRB 170817A. In addition, we show that the proposed scenario agrees with the Major Atmospheric Gamma-ray Imaging Cherenkov telescope, Fermi-Large Area Telescope, and High Energy Stereoscopic System upper limits on gamma-ray emission from GRB 160821B and GRB 170817A.

Modeling the Observations of GRB 180720B: from Radio to Sub-TeV Gamma-Rays

Abstract Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes, and emitting regions associated with the spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories at multiple wavelengths that range from radio bands to sub-TeV gamma-rays. The simultaneous multiwavelength observations were presented over multiple periods of time beginning just after the trigger time and extending to more than 30 days. The temporal and spectral analysis of Fermi Large Area Telescope (LAT) observations suggests that it presents similar characteristics to other bursts detected by this instrument. Coupled with X-ray and optical observations, the standard external shock model in a homogeneous medium is favored by this analysis. The X-ray flare is consistent with the synchrotron self-Compton (SSC) model from the reverse-shock region evolving in a thin shell and previous LAT, X-ray, and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with Markov chain Monte Carlo simulations indicate that the outflow is endowed with magnetic fields and that the radio observations are in the self-absorption regime. The SSC forward-shock model with our parameters can explain the LAT photons beyond the synchrotron limit as well as the emission recently reported by the HESS Collaboration.

Synchrotron Self-Compton Emission from External Shocks as the Origin of the Sub-TeV Emission in GRB 180720B and GRB 190114C

Abstract Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100–1000 s and 10 hr after the burst. We model the available broadband data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant-density circumburst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circumburst density and shock microphysical parameters) in the modeling are not unusual for both gamma-ray bursts, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.

Origin for the Prompt Spectral Evolution Characteristics and High Energy Emission during the X-Ray Flare in GRB 180720B

Abstract The gamma-ray burst GRB 180720B is very peculiar. On one hand, some interesting features have been found by performing the detailed time-resolved spectral analysis in the prompt phase. First, the “flux-tracking” pattern is exhibited both for the low energy spectral index α and the peak energy E p in the Band function. Second, some parameter relations show strong monotonous positive correlations, include E p –F, α–F, E p –α, and E p –L γ,iso for all time-resolved spectra. Lastly, it should be noted that the values of α do not exceed the synchrotron limits . On the other hand, the photons with the energy of ≳100 MeV were detected by LAT both in the prompt phase and afterglow. Notably, the 5 GeV photon was observed at 142 s after the GBM trigger. The spectrum of this burst in the LAT range can be described as F ν ∝ ν −1.3 t −1.54±0.02 in the afterglow phase. And there are six GeV photons during the X-ray flare when the lower energy emission is fading to a weaker level. We try to give reasonable interpretations of the mechanism for prompt emission and the high energy emission (100 MeV to GeV) in the afterglow. The interpretations suggesting that synchrotron origin can account for the prompt emission and synchrotron self-Compton radiation can account for both the spectrum and temporal behavior of the 100 MeV to GeV afterglow emission that have been accepted by us.

Fermi-GBM GRBs with Characteristics Similar to GRB 170817A

Abstract We present a search for gamma-ray bursts in the Fermi-GBM 10 yr catalog that show similar characteristics to GRB 170817A, the first electromagnetic counterpart to a GRB identified as a binary neutron star (BNS) merger via gravitational wave observations. Our search is focused on a nonthermal pulse, followed by a thermal component, as observed for GRB 170817A. We employ search methods based on the measured catalog parameters and Bayesian Block analysis. Our multipronged approach, which includes examination of the localization and spectral properties of the thermal component, yields a total of 13 candidates, including GRB 170817A and the previously reported similar burst, GRB 150101B. The similarity of the candidates is likely caused by the same processes that shaped the gamma-ray signal of GRB 170817A, thus providing evidence of a nearby sample of short GRBs resulting from BNS merger events. Some of the newly identified counterparts were observed by other space telescopes and ground observatories, but none of them have a measured redshift. We present an analysis of this subsample, and we discuss two models. From uncovering 13 candidates during a time period of 10 yr we predict that Fermi-GBM will trigger on-board on about one burst similar to GRB 170817A per year.

X-Ray Afterglows of Short Gamma-Ray Bursts: Magnetar or Fireball?

Abstract The origin of the X-ray afterglows of gamma-ray bursts has regularly been debated. We fit both the fireball-shock and millisecond-magnetar models of gamma-ray bursts to the X-ray data of GRB 130603B and 140903A. We use Bayesian model selection to answer the question of which model best explains the data. This is dependent on the maximum allowed non-rotating neutron star mass M TOV, which depends solely on the unknown nuclear equation of state. We show that the data for GRB 140903A favors the millisecond-magnetar model for all possible equations of state, while the data for GRB 130603B favors the millisecond-magnetar model if M TOV ≳ 2.3 M ⊙. If M TOV ≲ 2.3 M ⊙, the data for GRB 130603B supports the fireball-shock model. We discuss implications of this result in regards to the nuclear equation of state and the prospect of gravitational-wave emission from newly born millisecond magnetars.

GRB 170817A-GW170817-AT 2017gfo and the observations of NS-NS, NS-WD and WD-WD mergers

The LIGO-Virgo Collaboration has announced the detection of GW170817 and has associated it with GRB 170817A . These signals have been followed after 11 hours by the optical and infrared emission of AT 2017gfo. The origin of this complex phenomenon has been attributed to a neutron star-neutron star (NS-NS) merger. In order to probe this association we confront our current understanding of the gravitational waves and associated electromagnetic radiation with four observed GRBs originating in binaries composed of different combinations NSs and white dwarfs (WDs). We consider 1) GRB 090510 the prototype of NS-NS merger leading to a black hole (BH); 2) GRB 130603B the prototype of a NS-NS merger leading to massive NS (MNS) with an associated kilonova; 3) GRB 060614 the prototype of a NS-WD merger leading to a MNS with an associated kilonova candidate; 4) GRB 170817A the prototype of a WD-WD merger leading to massive WD with an associated AT 2017gfo-like emission. None of these systems support the above mentioned association. The clear association between GRB 170817A and AT 2017gfo has led to introduce a new model based on a new subfamily of GRBs originating from WD-WD mergers. We show how this novel model is in agreement with the exceptional observations in the optical, infrared, X- and gamma-rays of GRB 170817A-AT 2017gfo.