Prompt Emission Mechanism Research Articles

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.

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.

Optimizing the Resolution of Hydrodynamic Simulations for MCRaT Radiative Transfer Calculations

Despite their discovery about half a century ago, the gamma-ray burst (GRB) prompt emission mechanism is still not well understood. Theoretical modeling of the prompt emission has advanced considerably due to new computational tools and techniques. One such tool is the PLUTO hydrodynamics code, which is used to numerically simulate GRB outflows. PLUTO uses Adaptive Mesh Refinement to focus computational efforts on the portion of the grid that contains the simulated jet. Another tool is the Monte Carlo Radiation Transfer (MCRaT) code, which predicts the electromagnetic signatures of GRBs by conducting photon scatterings within a jet using PLUTO. The effects of the underlying resolution of a PLUTO simulation with respect to MCRaT post-processing radiative transfer results have not yet been quantified. We analyze an analytic spherical outflow and a hydrodynamically simulated GRB jet with MCRaT at varying spatial and temporal resolutions and quantify how decreasing both resolutions affects the resulting mock observations. We find that changing the spatial resolution changes the hydrodynamic properties of the jet, which directly affect the MCRaT mock observable peak energies. We also find that decreasing the temporal resolution artificially decreases the high-energy slope of the mock observed spectrum, which increases both the spectral peak energy and the luminosity. We show that the effects are additive when both spatial and temporal resolutions are modified. Our results allow us to understand how decreased hydrodynamic temporal and spatial resolutions affect the results of post-processing radiative transfer calculations, allowing for the optimization of hydrodynamic simulations for radiative transfer codes.

The Prompt Emission of GRB 130518A and the Study of Its Outflow through Hybrid Jet Models

The nature of the prompt emission mechanism in gamma-ray bursts (GRBs) remains uncertain to date. This question is also tied to the composition of the jet: thermal, Poynting flux dominated or hybrid with both types of components. In this work, we aim to study these questions in the context of GRB 130518A. By analysing its prompt emission data from the Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT), we find that the time-integrated spectrum has a nonthermal component along with a subdominant blackbody component. We use these results to study the properties of the jet in various scenarios through the frameworks developed by Hascoët et al. and Gao & Zhang. Both frameworks exclude the pure fireball model for this GRB. In all other cases, the initial magnetic fraction is greater than the thermal fraction. For small launching radii of the jet, the favored nonthermal emission process is internal shocks. Magnetic reconnection seems likely only for very large initial radii.

Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts

Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100 - 600 keV for the GRBs detected in the first year of operation. CZTI in last five years has detected around 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

Evaluation of a one-dimensional position-sensitive quartz optical fiber sensor based on the time-of-flight method for high radiation dose rate applications

For the measurement of radiation distribution inside the Fukushima Daiichi Nuclear Power Station (FDNPS) buildings, the evaluation of a small-diameter quartz optical fiber as a one-dimensional position-sensitive sensor was conducted. The sensor determines the incident position of radiation into the fiber using the time-of-flight information from the Cerenkov photons generated in the optical fiber. Compared with a conventional sensor using plastic scintillating fiber, the quartz optical fiber has a much higher position resolution, which may be the result of the improvement of timing characteristics due to the prompt emission mechanism of the Cerenkov radiation. Additionally, the response of the quartz fiber sensor under high radiation field was evaluated. A count rate linearity up to a dose rate of at least 20 mSv/h was confirmed using a 10m-long quartz fiber with a diameter of 0.4 mm. Furthermore, the radiation tolerance of the sensor up to an accumulated dose of 1 kGy was evaluated.

TOWARD AN UNDERSTANDING OF GRB PROMPT EMISSION MECHANISM: II. PATTERNS OF PEAK ENERGY EVOLUTION AND THEIR CONNECTION TO SPECTRAL LAGS.

The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinct patterns of the peak-energy (E p ) evolution; i.e., time-resolved spectral analyses of νF ν spectra of broad pulses reveal (1) "hard-to-soft" and (2) "flux-tracking" patterns of E p evolution in time, the physical origin of which still remains not well understood. We show here that these two patterns can be successfully reproduced within a simple physical model invoking synchrotron radiation in a bulk-accelerating emission region. We show further that the evolution patterns of the peak energy have, in fact, direct connections to the existence of two different (positive or negative) types of spectral lags, seen in the broad pulses. In particular, we predict that (1) only the positive type of spectral lags is possible for the hard-to-soft evolution of the peak energy, (2) both the positive and negative type of spectral lags can occur in the case of flux-tracking pattern of the peak energy, (3) for the flux-tracking pattern, the peak location of the flux light curve slightly lags behind the peak of the E p evolution with time if the spectral lags are positive, and (4) in the case of flux-tracking pattern, double-peaked broad pulses can appear in the light curves, the shape of which is energy-dependent.

Detection of the thermal component in GRB 160107A

Abstract We present the detection of a blackbody component in gamma-ray burst GRB 160107A emission by using the combined spectral data of the CALET Gamma-ray Burst Monitor (CGBM) and the MAXI Gas Slit Camera (GSC). MAXI/GSC detected the emission ∼45 s prior to the main burst episode observed by the CGBM. The MAXI/GSC and the CGBM spectrum of this prior emission period is fitted well by a blackbody with temperature $1.0^{+0.3}_{-0.2}$ keV plus a power law with a photon index of −1.6 ± 0.3. We discuss the radius of the photospheric emission and the main burst emission based on the observational properties. We stress the importance of coordinated observations via various instruments collecting high-quality data over a broad energy coverage in order to understand the GRB prompt emission mechanism.

Gamma-Ray Bursts and Population III Stars

Gamma-ray bursts (GRBs) are ideal probes of the epoch of the first stars and galaxies. We review the recent theoretical understanding of the formation and evolution of the first (so-called Population III) stars, in light of their viability of providing GRB progenitors. We proceed to discuss possible unique observational signatures of such bursts, based on the current formation scenario of long GRBs. These include signatures related to the prompt emission mechanism, as well as to the afterglow radiation, where the surrounding intergalactic medium might imprint a telltale absorption spectrum. We emphasize important remaining uncertainties in our emerging theoretical framework.

HOT ELECTROMAGNETIC OUTFLOWS. III. DISPLACED FIREBALL IN A STRONG MAGNETIC FIELD

The evolution of a dilute electron-positron fireball is calculated in the regime of strong magnetization and very high compactness (l ~10^3-10^8). Heating is applied at a low effective temperature (< 25 keV), and the fireball is allowed to expand, so that the formation of a black-body spectral distribution is inhibited by pair annihilation. The diffusion equation for Compton scattering is coupled to a single-temperature pair gas and an exact (trans-relativistic) cyclo-synchrotron photon source. We find that the photon spectrum develops a quasi-thermal peak, with a power-law slope below it that is characteristic of gamma-ray bursts. The formation of a thermal high-frequency spectrum is checked using the full kinetic equations at l ~ 10^3. These results have several implications for the central engine of GRBs, and the mechanism of energy transport. 1. Baryon rest mass carries less than ~ 10^{-5} of the energy flux at jet breakout inside ~ 10^{12} cm from the engine, with most carried by the magnetic field. 2. This degree of baryon purity points to the presence of an event horizon in the engine, and neutrons play a negligible role in the prompt emission mechanism. 3. X-ray flashes are emitted by outflows carrying enough baryons that the photosphere is pair-depleted, which we show results in faster thermalization. 4. The relation between observed peak frequency and burst luminosity is bounded below by the observed Amati et al. relation if jet Lorentz factor ~ 1/(opening angle) at breakout. 5. Stellar models are used to demonstrate an inconsistency between the highest observed GRB energies, and a hydrodynamic nozzle: magnetic collimation is required. 6. The magnetized pair gas is dilute enough that high-frequency Alfven waves may become charge starved. Finally, we suggest that limitations on magnetic reconnection from plasma collisionality have been overestimated.

NEWFERMI-LAT EVENT RECONSTRUCTION REVEALS MORE HIGH-ENERGY GAMMA RAYS FROM GAMMA-RAY BURSTS

Based on the experience gained during the four and a half years of the mission, the Fermi -LAT collaboration has undertaken a comprehensive revision of the event-level analysis going under the name of Pass 8. Although it is not yet finalized, we can test the improvements in the new event reconstruction with the special case of the prompt phase of bright Gamma-Ray Bursts (GRBs), where the signal to noise ratio is large enough that loose selection cuts are sufficient to identify gamma- rays associated with the source. Using the new event reconstruction, we have re-analyzed ten GRBs previously detected by the LAT for which an x-ray/optical follow-up was possible and found four new gamma rays with energies greater than 10 GeV in addition to the seven previously known. Among these four is a 27.4 GeV gamma-ray from GRB 080916C, which has a redshift of 4.35, thus making it the gamma ray with the highest intrinsic energy (147 GeV) detected from a GRB. We present here the salient aspects of the new event reconstruction and discuss the scientific implications of these new high-energy gamma rays, such as constraining extragalactic background light models, Lorentz invariance violation (LIV) tests, the prompt emission mechanism and the bulk Lorentz factor of the emitting region.

GAMMA-RAY POLARIZATION INDUCED BY COLD ELECTRONS VIA COMPTON PROCESSES

The polarization measurement is an important tool to probe the prompt emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be scattered by cold electrons in the outflow via Compton scattering processes. The observed polarization depends on both the photon energy and the viewing angle. With the typical bulk Lorentz factor $\Gamma \sim 200$, photons with energy $E>10$ MeV tend to have smaller polarization than photons with energy $E<1$ MeV. At the right viewing angle, i.e. $\theta \sim \Gamma^{-1}$, the polarization achieves its maximal value, and the polarization angle changes $90^{\circ}$ relative to the initial polarization direction. Thus, the synchrotron radiation plus Compton scattering model can naturally explain the $90^{\circ}$ change of the polarization angle in GRB 100826A.

RHESSITESTS OF QUASI-THERMAL GAMMA-RAY BURST SPECTRAL MODELS

Prompt gamma-ray burst (GRB) spectra evolve on short timescales, suggesting that time-resolved spectral fits may help diagnose the still unknown prompt emission mechanism. We use broadband gamma-ray data from the Ramaty High-Energy Solar Spectroscopic Imager spacecraft to test quasi-thermal models with high signal-to-noise time-resolved spectra of nine bright GRBs. In contrast to results reported in more narrow energy bands, the quality of the fits of quasi-thermal models is poor in relation to fits of the phenomenological Band function. Moreover, the best-fit parameters for the simplest quasi-thermal model, a blackbody plus a nonthermal power law, show significant dependence on the fit band. Models that replace the power law with more complicated nonthermal functions are not robust for the data considered here and decrease the physical relevance of the fit blackbody.

X-ray flares in early GRB afterglows

The Swift X-ray Telescope (XRT) has discovered that flares are quite common in early X-ray afterglows of gamma-ray bursts (GRBs), being observed in roughly 50% of afterglows with prompt follow-up observations. The flares range in fluence from a few per cent to approximately 100% of the fluence of the prompt emission (the GRB). Repetitive flares are seen, with more than four successive flares detected by the XRT in some afterglows. The rise and fall times of the flares are typically considerably smaller than the time since the burst. These characteristics suggest that the flares are related to the prompt emission mechanism, but at lower photon energies. We conclude that the most likely cause of these flares is late-time activity of the GRB central engine.