Prompt Emission Models Research Articles

Model Constraints Based on the IceCube Neutrino Nondetection of GRB 221009A

GRB 221009A is a bright gamma-ray burst (GRB) with isotropic energy larger than 1054 erg. Its fairly low redshift makes it a promising candidate for high-energy neutrino detection. However, a neutrino search for this GRB reported by the IceCube collaboration yielded a null result. In this paper, we utilize the upper limit from the IceCube observation to test different GRB prompt emission models. We find that, at least for this specific burst, the dissipative photosphere model could be ruled out in a large parameter space. The internal-shock model can survive only with a large bulk motion Lorentz factor Γ, where the most stringent and conservative constraints are Γ > ∼ 450 and Γ > ∼ 200, respectively. Also, the ratio of the total dissipated energy that goes into the protons and electrons (ϵ p /ϵ e ) can be constrained with a given Γ. For Γ < 400, ϵ p /ϵ e < 10 is required. For the Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) model, the constraint from GRB 221009A is modest. Under the ICMART model, only for extreme situations when most dissipated energy deposit into protons and all accelerated protons are suitable for producing neutrinos, a slightly large bulk motion (Γ > ∼ 250) is required.

GRB Prompt Emission: Observed Correlations and Their Interpretations

The prompt emission of Gamma Ray Bursts (GRBs) is still an outstanding question in the study of these cataclysmic events. Part of what makes GRBs difficult to study is how unique each event seems to be. However, aggregating many GRB observations and analyzing the population allows us to obtain a better understanding of the emission mechanism that produces the observed prompt emission. In this review, we outline some of the most prevalent correlations that have emerged from GRB prompt emission observations and how these correlations are interpreted in relation to physical properties and prompt emission models of GRB.

Photospheric Prompt Emission From Long Gamma-ray Burst Simulations. I. Optical Emission

Abstract A complete understanding of gamma-ray bursts (GRBs) has been difficult to achieve, due to our incomplete knowledge of the radiation mechanism that is responsible for producing the prompt emission. This emission, which is detected in the first tens of seconds of the GRB, is typically dominated by hard X-ray and gamma-ray photons, although there have also been a few dozen prompt optical detections. These optical detections have the potential to discriminate between plausible prompt emission models, such as the photospheric and synchrotron shock models. In this work, we use an improved MCRaT code, which includes cyclo-synchrotron emission and absorption, to conduct radiative transfer calculations from optical to gamma-ray energies under the photospheric model. The calculations are conducted using a set of two-dimensional relativistic hydrodynamic long GRB jet simulations, consisting of a constant and a variable jet. We predict the correlations between the optical and gamma-ray light curves as functions of observer angle and jet variability, and find that there should be extremely dim optical prompt precursors for large viewing angles. Additionally, the detected optical emission originates from dense regions of the outflow, such as shock interfaces and the jet-cocoon interface. Our results also show that the photospheric model is unable to account for the current set of optical prompt detections that have been made and therefore additional radiative mechanisms are needed to explain these prompt optical observations. These findings show the importance of conducting global radiative transfer simulations using hydrodynamically calculated jet structures.

Murchison Widefield Array rapid-response observations of the short GRB 180805A

AbstractHere we present stringent low-frequency (185 MHz) limits on coherent radio emission associated with a short-duration gamma-ray burst (SGRB). Our observations of the short gamma-ray burst (GRB) 180805A were taken with the upgraded Murchison Widefield Array (MWA) rapid-response system, which triggered within 20s of receiving the transient alert from theSwiftBurst Alert Telescope, corresponding to 83.7 s post-burst. The SGRB was observed for a total of 30 min, resulting in a$3\sigma$persistent flux density upper limit of 40.2 mJy beam–1. Transient searches were conducted at theSwiftposition of this GRB on 0.5 s, 5 s, 30 s and 2 min timescales, resulting in$3\sigma$limits of 570–1 830, 270–630, 200–420, and 100–200 mJy beam–1, respectively. We also performed a dedispersion search for prompt signals at the position of the SGRB with a temporal and spectral resolution of 0.5 s and 1.28 MHz, respectively, resulting in a$6\sigma$fluence upper-limit range from 570 Jy ms at DM$=3\,000$pc cm–3($z\sim 2.5$) to 1 750 Jy ms at DM$=200$pc cm–3($z\sim 0.1)$, corresponding to the known redshift range of SGRBs. We compare the fluence prompt emission limit and the persistent upper limit to SGRB coherent emission models assuming the merger resulted in a stable magnetar remnant. Our observations were not sensitive enough to detect prompt emission associated with the alignment of magnetic fields of a binary neutron star just prior to the merger, from the interaction between the relativistic jet and the interstellar medium (ISM) or persistent pulsar-like emission from the spin-down of the magnetar. However, in the case of a more powerful SGRB (a gamma-ray fluence an order of magnitude higher than GRB 180805A and/or a brighter X-ray counterpart), our MWA observations may be sensitive enough to detect coherent radio emission from the jet-ISM interaction and/or the magnetar remnant. Finally, we demonstrate that of all current low- frequency radio telescopes, only the MWA has the sensitivity and response times capable of probing prompt emission models associated with the initial SGRB merger event.

Photospheric Polarization Signatures from Long Gamma-Ray Burst Simulations

Abstract A comprehensive understanding of gamma-ray bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two-dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carlo Radiation Transport (MCRaT) code. MCRaT has been enhanced by the inclusion of polarization; it has been first verified by reproducing a variety of results in the literature and then used to obtain the time-integrated and time-resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees (≲1%) are consistent with the constraints from the POLAR experiment, they are lower than other theoretical studies due to the lack of strong gradients in the model jet profiles that we use. The time-resolved results suggests that GRBs with wide jets observed on axis will have small polarization degrees (≲2%) and constant polarization angles, during the brightest portion of the light curve. GRBs observed off-axis will have larger polarization degrees and polarization angles that change with the temporal structure of radiating shells in the outflow. We then place our results in the context of GRB prompt emission models and future LEAP and POLAR-2 GRB polarimetry detections.

Violation of Synchrotron Line of Death by the Highly Polarized GRB 160802A

Abstract GRB 160802A is one of the brightest gamma-ray bursts (GRBs) observed by the Fermi Gamma-ray Burst Monitor (GBM) in the energy range of 10–1000 keV, while at the same time it is surprisingly faint at energies ≳2 MeV. An observation with the AstroSat/CZT Imager also provides the polarization that helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the Fermi/GBM data, and find two main bursting episodes that are clearly separated in time, one of which is particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behavior corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90% cases, crosses even the slow cooling limit (α = −2/3) of an optically thin synchrotron shock model. Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarization in 100–300 keV is too high, π = 85 ± 29%, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to α = +0.5, in principle can produce high polarization, but only beyond the spectral peak, which in our case lies close to 200–300 keV during the time when most of the polarization signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

Revisiting the residual temperature distribution in prompt neutron emission in fission

A new triangular form of the residual temperature distribution P(T), entering the prompt emission models in which the sequential emission is globally taken into account (e.g., the Los Alamos model of Madland and Nix with subsequent improvements and the Point-by-Point model), is proposed. A deterministic treatment of the successive emission of prompt neutrons, which is based on recursive equations of the residual temperatures, was developed. This modeling was validated by the good description of many and different experimental data of prompt emission (e.g., $\overline{\nu}(A)$ , $\langle\nu\rangle$ (TKE), $\langle\varepsilon\rangle (A)$ , $\langle\varepsilon\rangle$ (TKE), $\overline{E}_{\gamma} (A)$ , etc.) and the good agreement with the results of other prompt emission models. To see a possible systematic behaviour of P(T) as a function of energy and fissioning nucleus, the deterministic treatment of sequential emission was applied to 11 nuclei undergoing fission (spontaneously or induced by thermal and fast neutrons with energies up to the threshold of the second chance fission) for which reliable experimental fission fragment distributions Y(A, TKE) exist. The shapes of all P(T) distributions for the light and heavy fragment groups and for all fragments resulting from this modeling can be approximated with a triangular form. To make possible the use of this form into the prompt emission models with a global treatment of sequential emission, a connection between the average residual temperature $\langle \mathrm{Tr} \rangle$ and the temperature of initial fragments $\langle \mathrm{Ti} \rangle$ is needed. An important finding of this study concerns the ratio $\langle \mathrm{Tr} \rangle / \langle \mathrm{Ti} \rangle$ , which is $ \approx 0.6$ for all studied fissioning systems. This result allows to obtain a new triangular form of P(T) defined only as a function of initial temperature, which is applicable to any fissioning system at any energy, in the frame of prompt emission models with a global treatment of sequential emission.

Constraints on millisecond magnetars as the engines of prompt emission in gamma-ray bursts

We examine millisecond magnetars as central engines of Gamma Ray Bursts' (GRB) prompt emission. Using the proto-magnetar wind model of Metzger et al. 2011, we estimate the temporal evolution of the magnetization and power injection at the base of the GRB jet and apply these to different prompt emission models to make predictions for the GRB energetics, spectra and lightcurves. We investigate both shock and magnetic reconnection models for the particle acceleration, as well as the effects of energy dissipation across optically thick and thin regions of the jet. The magnetization at the base of the jet, $\sigma_0$, is the main parameter driving the GRB evolution in the magnetar model and the emission is typically released for $100\lesssim \sigma_0 \lesssim 3000$. Given the rapid increase in $\sigma_0$ as the proto-magnetar cools and its neutrino-driven mass loss subsides, the GRB duration is typically limited to $\lesssim 100$ s. This low baryon loading at late times challenges magnetar models for ultra-long GRBs, though black hole models likely run into similar difficulties without substantial entrainment from the jet walls. The maximum radiated gamma-ray energy is $\lesssim 5 \times 10^{51}$erg, significantly less than the magnetar's total initial rotational energy and in strong tension with the high end of the observed GRB energy distribution. However, the gradual magnetic dissipation model (Beniamini & Giannios 2017) applied to a magnetar central engine, naturally explains several key observables of typical GRBs, including energetics, durations, stable peak energies, spectral slopes and a hard to soft evolution during the burst.

Comprehensive overview of the Point-by-Point model of prompt emission in fission

The investigation of prompt emission in fission is very important in understanding the fission process and to improve the quality of evaluated nuclear data required for new applications. In the last decade remarkable efforts were done for both the development of prompt emission models and the experimental investigation of the properties of fission fragments and the prompt neutrons and $\gamma$ -ray emission. The accurate experimental data concerning the prompt neutron multiplicity as a function of fragment mass and total kinetic energy for 252Cf(SF) and 235 (n, f) recently measured at JRC-Geel (as well as other various prompt emission data) allow a consistent and very detailed validation of the Point-by-Point (PbP) deterministic model of prompt emission. The PbP model results describe very well a large variety of experimental data starting from the multi-parametric matrices of prompt neutron multiplicity $\nu (A,TKE)$ and $\gamma$ -ray energy $E_{\gamma}(A,TKE)$ which validate the model itself, passing through different average prompt emission quantities as a function of A (e.g., $\nu(A)$ , $E_{\gamma}(A)$ , $\langle \varepsilon \rangle (A)$ etc.), as a function of TKE (e.g., $\nu (TKE)$ , $E_{\gamma}(TKE)$ ) up to the prompt neutron distribution $P (\nu)$ and the total average prompt neutron spectrum. The PbP model does not use free or adjustable parameters. To calculate the multi-parametric matrices it needs only data included in the reference input parameter library RIPL of IAEA. To provide average prompt emission quantities as a function of A, of TKE and total average quantities the multi-parametric matrices are averaged over reliable experimental fragment distributions. The PbP results are also in agreement with the results of the Monte Carlo prompt emission codes FIFRELIN, CGMF and FREYA. The good description of a large variety of experimental data proves the capability of the PbP model to be used in nuclear data evaluations and its reliability to predict prompt emission data for fissioning nuclei and incident energies for which the experimental information is completely missing. The PbP treatment can also provide input parameters of the improved Los Alamos model with non-equal residual temperature distributions recently reported by Madland and Kahler, especially for fissioning nuclei without any experimental information concerning the prompt emission.

Point-by-Point model calculation of the prompt neutron multiplicity distributionν(A)forU238(n,f)at incident neutron energies ranging from 1 MeV to 80 MeV

Prompt neutron multiplicity distributions \ensuremath{\nu}($A$) are generally required for prompt emission correction of double energy ($2E$) measurements of fission fragments in order to determine pre-neutron fragment properties. The lack of experimental \ensuremath{\nu}($A$) data especially at higher incident neutron energy imposes the use of prompt emission models to predict \ensuremath{\nu}($A$). At incident neutron energies ($\mathit{En}$) where multiple fission chances are involved, the Point-by-Point (PbP) model of prompt emission is able to provide the individual \ensuremath{\nu}($A$) of the compound nuclei of the main and secondary nucleus chains that are undergoing fission at a given $\mathit{En}$. The total \ensuremath{\nu}($A$) are obtained by averaging these individual \ensuremath{\nu}($A$) over the fission chance probabilities (expressed as total and partial fission cross-section ratios). An indirect validation of the total \ensuremath{\nu}($A$) results is proposed. At high $\mathit{En}$ (above 70 MeV) the PbP results of individual \ensuremath{\nu}($A$) of the first few nuclei of the main and secondary nucleus chains exhibit an almost linear increase. This shape is explained by the damping of shell effects entering the superfluid expression of the level-density parameters. They tend to approach the asymptotic values for a great part of the fragments. This fact leads to a smooth and almost linear increase of fragment excitation energy with the fragment mass number that is reflected in a smooth and almost linear behavior of individual \ensuremath{\nu}($A$). The comparison of the present results with those of the GEF code reveals different shapes of \ensuremath{\nu}($A$) as well as different total average neutron multiplicity as a function of the $\mathit{En}$. At high $\mathit{En}$ the PbP calculations definitely reflect the influence of the almost linear shape of individual \ensuremath{\nu}($A$) of the first few nuclei of the U and Pa chains. The differences between the total \ensuremath{\nu}($A$) obtained by averaging the PbP results of individual \ensuremath{\nu}($A$) over fission cross-section ratios of different evaluations are insignificant.

GRAVITATIONAL-WAVE OBSERVATIONS MAY CONSTRAIN GAMMA-RAY BURST MODELS: THE CASE OF GW150914–GBM

ABSTRACT The possible short gamma-ray burst (GRB) observed by Fermi/GBM in coincidence with the first gravitational-wave (GW) detection offers new ways to test GRB prompt emission models. GW observations provide previously inaccessible physical parameters for the black hole central engine such as its horizon radius and rotation parameter. Using a minimum jet launching radius from the Advanced LIGO measurement of GW 150914, we calculate photospheric and internal shock models and find that they are marginally inconsistent with the GBM data, but cannot be definitely ruled out. Dissipative photosphere models, however, have no problem explaining the observations. Based on the peak energy and the observed flux, we find that the external shock model gives a natural explanation, suggesting a low interstellar density (∼10−3 cm−3) and a high Lorentz factor (∼2000). We only speculate on the exact nature of the system producing the gamma-rays, and study the parameter space of a generic Blandford–Znajek model. If future joint observations confirm the GW–short-GRB association we can provide similar but more detailed tests for prompt emission models.

Synchrotron cooling in energetic gamma-ray bursts observed by theFermiGamma-Ray Burst Monitor

We study the time-resolved spectra of eight GRBs observed by Fermi GBM in its first five years of mission, with 1 keV - 1 MeV fluence $f>1.0\times10^{-4}$ erg cm$^{-2}$ and signal-to-noise level $\text{S/N}\geq10.0$ above 900 keV. We aim to constrain in detail the spectral properties of GRB prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models. We perform time-resolved spectral analysis using a variable temporal binning technique according to optimal S/N criteria, resulting in a total of 299 time-resolved spectra. We fit the Band function to all spectra and obtain the distributions for the low-energy power-law index $\alpha$, the high-energy power-law index $\beta$, the peak energy in the observed $\nu F_\nu$ spectrum $E_\text{p}$, and the difference between the low- and high-energy power-law indices $\Delta s=\alpha-\beta$. Using the distributions of $\Delta s$ and $\beta$, the electron population index $p$ is found to be consistent with the "moderately fast" scenario which fast- and slow-cooling scenarios cannot be distinguished. We also apply a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test for consistency with a synchrotron origin for the prompt emission and obtain the distributions for the two break energies $E_\text{b,1}$ and $E_\text{b,2}$, the middle segment power-law index $\beta$, and the Planck function temperature $kT$. A synchrotron model is found consistent with the majority of time-resolved spectra for these eight energetic Fermi GBM bursts with good high-energy photon statistics, as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by inclusion of a thermal component or when an evolving magnetic field is accounted for.

Realistic analytic model for the prompt and high-latitude emission in GRBs

Most gamma-ray bursts (GRBs) observed by the Swift satellite show an early rapid decay phase (RDP) in their X-ray lightcurve, which is usually a smooth continuation of the prompt gamma-ray emission, strongly suggesting that it is its tail. However, the mechanism behind it is still not clear. The most popular model for this RDP is High Latitude Emission (HLE). While HLE is expected in many models for the prompt GRB emission, such as the popular internal shocks model, there are models in which it is not expected, such as sporadic magnetic reconnection events. Therefore, testing whether the RDP is consistent with HLE can help distinguish between different prompt emission models. We address this question by modeling the prompt emission as the sum of its individual pulses with their HLE tails. Analytic expressions for the observed flux density are obtained for power-law and Band function emission spectra. For internal shocks the observed instantaneous spectrum is very close to the emitted one, and should be well described by a Band function also during the RDP. Our model naturally produces, the observed spectral softening and steepening of the flux decay. The observed flux during the RDP is initially dominated by the tail of the last pulse, but the tails of one or more earlier pulses can become dominant later on. Moreover, modeling several overlapping pulses as a single wider pulse would over-predict the emission tail. Thus, one should be very careful when testing the predictions of HLE and do a combined temporal and spectral fit of the prompt GRB emission and the RDP.

Some Recent Developments in γ-ray Burst Afterglow and Prompt Emission Models

Extensive observational campaigns of afterglow hunting have greatly enriched our understanding of the gamma-ray burst (GRB) phenomenon. Efforts have been made recently to explore some afterglow properties or signatures that will be tested by the on-going or the future observational campaigns yet come. These include the properties of GRB early afterglows in the temporal domain; the GeV-TeV afterglow signatures in the spectral domain; as well as a global view about the GRB universal structured jet configuration. These recent efforts are reviewed. Within the standard cosmological fireball model, the very model(s) responsible for the GRB prompt emission is (are) not identified. These models are critically reviewed and confronted with the current data.