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Register a new userA backscattering dominated prompt emission model for the prompt phase of Gamma ray bursts
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As gamma-ray burst (GRB) jet drills its way through the collapsing star, it traps a baryonic cork ahead of it. Here we explore a prompt emission model for GRBs in which the jet does not cross the cork, but rather photons that are emitted deep in the flow largely by pair annihilation are scattered inside the expanding cork and escape largely from the back end of it as they push it from behind. Due to the relativistic motion of the cork, these photons are easily seen by an observer close to the jet axis peaking at $varepsilon_{peak}sim~few times 100 keV$. We show that this model naturally explains several key observational features including: (1) High energy power law index $beta_1 sim -2 {~rm to~} -5$ with an intermediate thermal spectral region; (2) decay of the prompt emission light curve as $sim t^{-2}$; (3) Delay of soft photons; (4) peak energy - isotropic energy (the so-called Amati) correlation, $varepsilon_{peak} sim varepsilon_{iso}^m$, with $msim 0.45$, resulting from different viewing angles. At low luminosities, our model predicts an observable turn off in the Amati relation. (4) An anti-correlation between the spectral full width half maxima (FWHM) and time as $t^{-1}$. (5) Temporal evolution $varepsilon_{peak} sim t^{-1}$, accompanied by an increase of the high energy spectral slope with time. (6) Distribution of peak energies $varepsilon_{peak}$ in the observed GRB population. The model is applicable for a single pulse GRB lightcurves and respective spectra. We discuss the consequence of our model in view of the current and future prompt emission observations.
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Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GRB sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes.
GRB spectra appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central engine becomes transparent to its own radiation, with a quasi-blackbody spectrum in absence of additional sub-photospheric dissipation. However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. We aim to compute the thermal and non-thermal emissions produced by an outflow with a variable Lorentz factor, where the power injected at the origin is partially thermal (fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The thermal emission is produced at the photosphere, and the non-thermal emission in the optically thin regime. Apart from the value of epsilon_th, we want to test how the other model parameters affect the observed ratio of the thermal to non-thermal emission. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a thermal component in GRB spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these observations can be used to constrain the nature and acceleration mechanism of GRB outflows.
We present the first global model of prompt emission from a short gamma-ray burst that consistently describes the evolution of the central black-hole (BH) torus system, the propagation of the jet through multi-component merger ejecta, the transition into free expansion, and the photospheric emission from the relativistic jet. To this end, we perform a special relativistic neutrino-hydrodynamics simulation of a viscous BH-torus system, which is formed about 500ms after the merger and is surrounded by dynamical ejecta as well as neutron star winds, along with a jet that is injected in the vicinity of the central BH. In a post-processing step, we compute the photospheric emission using a relativistic Monte-Carlo radiative transfer code. It is found that the wind from the torus leaves a strong imprint on the jet as well as on the emission causing narrow collimation and rapid time variability. The viewing angle dependence of the emission gives rise to correlations among the spectral peak energy, E_p, isotropic energy, E_iso, and peak luminosity, L_p, which may provide natural explanations for the Amati- and Yonetoku-relations. We also find that the degree of polarization is small for the emission from the jet core (<2%), while it tends to increase with viewing angle outside of the core and can become as high as ~10-40% for energies larger than the peak energy. Finally, the comparison of our model with GRB170817A strongly disfavors the photospheric emission scenario and therefore supports alternative scenarios, such as the cocoon shock breakout.
We propose a model for the spectral formation of Gamma Ray Burst (GRB) prompt emission, where the phenomenological Bands function is usually applied to describe the GRB prompt emission. We suggest that the GRB prompt emission is mainly a result of two upscattering processes. The first process is the Comptonization of relatively cold soft photons of the star off electrons of a hot shell of plasma of temperature T_e of the order of 10^{9} K (or kT_e~100 keV) that moves sub-relativistically with the bulk velocity V_b substantially less than the speed of light c. In this phase, the Comptonization parameter Y is high and the interaction between a blackbody-like soft seed photon population and hot electrons leads to formation of a saturated Comptonization spectrum modified by the sub-relativistic bulk outflow. The second process is an upscattering of the previously Comptonized spectrum by the plasma outflow once it becomes relativistic. This process gives rise to the high-energy power-law component above the peak in the EF(E)-diagram where F(E) is the energy flux. The latter process can be described by a convolution of the Comptonized spectrum with a broken-power-law Green function. Possible physical scenarios for this second upscattering process are discussed. In the framework of our model, we give an interpretation of the Amati relation between the intrinsic spectral peak photon energy and radiated energy or luminosity, and we propose a possible explanation of the GRB temporal variability.
The prompt emission of gamma-ray bursts (GRBs) is widely thought to be radiation from accelerated electrons, but an appreciably larger amount of energy could be carried by accelerated protons, particularly if GRBs are the sources of ultra-high-energy cosmic rays (UHECRs). We model the expected photon spectra for such proton-dominated GRBs in the internal shock scenario through Monte Carlo simulations, accounting for various processes related to high-energy electrons and protons. Besides proton and muon synchrotron components, emission from photomeson-induced secondary pair cascades becomes crucial, generally enhancing the GeV-TeV and/or eV-keV photons and offering a signature of UHE protons. In some cases, it can overwhelm the primary electron component and result in GRBs peaking in the 10 MeV - 1 GeV range, which may be relevant to some bursts discussed in a recent re-analysis of EGRET TASC data. The dependence of the spectra on key quantities such as the bulk Lorentz factor, magnetic field and proton-to-electron ratio is nontrivial due to the nonlinear nature of cascading and the interplay of electron- and proton-induced components. Observations by {it Fermi}, ground-based telescopes and other facilities should test these expectations and provide critical constraints on the proton acceleration efficiency.
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