Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

Abstract

Even though the observed spectra for GRB prompt emission is well constrained, no single radiation mechanism can robustly explain its distinct non-thermal nature.Here we explore the radiation mechanism with the photospheric emission model using our Monte Carlo Radiative Transfer (MCRaT) code. We study the sub-photospheric Comptonization of fast cooled synchrotron photons while the Maxwellian electrons and mono-energetic protons are accelerated to relativistic energies by repeated dissipation events. Unlike previous simulations, we implement a realistic photon to electron number ratio N γ /N e ∼ 10 5 consistent with the observed radiative efficiency of a few percent. We show that it is necessary to have a critical number of episodic energy injection events N rh,cr ∼ few 10s − 100 in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy E inj,cr ∼ 2500 − 4000 m e c 2 per electron and produce an output photon spectrum consistent with observations. The observed GRB spectrum can be generated when the electrons are repeatedly accelerated to highly relativistic energies γ e,in ∼ few 10s − 100 in a jet with bulk Lorentz factor Γ ∼ 30 − 100, starting out from moderate optical depths τ in ∼ 20 − 40. The shape of the photon spectrum is independent of the initial photon energy distribution and baryonic energy content of the jet and hence independent of the emission mechanism, as expected for photospheric emission.

Diffusive photospheres in gamma-ray bursts

“…The main assumption of that work is the presence of unknown dissipation mechanism, which transforms part of the kinetic energy of the outflow into radiation, postulated in (Rees & Mészáros 2005). Such dissipation can boost the luminosity of thermal emission and it might be required to explain subdominant thermal component during the prompt emission or even the prompt emission itself, see (Bhattacharya & Kumar 2020). Concerning observations of this component in the early afterglow, dissipation is not required, as it is much weaker than the prompt radiation.…”

Electromagnetic counterparts of compact binary mergers

“…observed short-duration (10 −3 -10 3 s) prompt emission seen in the keV-MeV energy range (Meszaros, Rees & Papathanassiou 1994;Daigne & Mochkovitch 1998; or, alternatively, the jet energy can be dissipated below the photosphere (R ∼ 10 12 cm) and released at the photosphere (e.g. Rees & Mészáros 2005;Pe'er 2008;Beloborodov 2010;Lazzati et al 2013;Bhattacharya & Kumar 2020).…”

“…Alternatively, the prompt emission can be released at the photosphere, from subphotospheric dissipation (e.g. Rees & Mészáros 2005;Pe'er 2008;Beloborodov 2010;Lazzati et al 2013;Bhattacharya & Kumar 2020). The subphotospheric dissipation can proceed through different mechanisms.…”

“…via Comptonization below the GRB fireball reaches its transparency, mixed thermal plus non-thermal processes and inverse Compton reprocessing of softer photon field (e.g. Liang et al 1997;Blinnikov, Kozyreva & Panchenko 1999;Rees & Mészáros 2005;Giannios 2006;Pe'er 2008;Beloborodov 2010;Chhotray & Lazzati 2015;Vurm & Beloborodov 2016;Bhattacharya & Kumar 2020). For the second class of models (studies that consider synchrotron radiation above the photosphere), effects producing a hardening of the low-energy spectral index have been invoked, such as (1) effect of an energy dependent inverse Compton radiation (Klein-Nishina regime) (Derishev, Kocharovsky & Kocharovsky 2001), or self-absorption effect (Lloyd & Petrosian 2000), (2) effects of the magnetic field profile within the emitting region (Uhm & Zhang 2014), (3) the non-uniform small-scale magnetic fields (Medvedev 2000), (4) adiabatic cooling of electrons (Geng et al 2018;Panaitescu 2019), (5) reacceleration and slow or balanced heating of the cooling electrons (Kumar & McMahon 2008;Asano & Terasawa 2009;Beniamini, Barniol Duran & Giannios 2018;Xu, Yang & Zhang 2018) (6) supercritical regime of hadronic cascades, i.e.…”

GRB spectrum from gradual dissipation in a magnetized outflow

“…In this scenario, the spectral peak and the low-energy part of the spectrum at photon energies E < E pk are formed by quasithermal Comptonization of soft seed photons up to the thermal peak by mildly relativistic electrons when the flow is optically thick with Thomson optical depth 1 τ T 100 (e.g. Eichler & Levinson 2000;Pe'er & Waxman 2004;Rees & Mészáros 2005;Giannios & Spruit 2007;Beloborodov 2013;Vurm, Lyubarsky & Piran 2013;Bhattacharya & Kumar 2020). Continued dissipation as the flow becomes optically thin (τ T < 1) then gives rise to the high-energy part of the spectrum at photon energies E > E pk (e.g.…”