Monte Carlo simulations of the photospheric process

Abstract

We present a Monte Carlo (MC) code we wrote to simulate the photospheric process and to study the photospheric spectrum above the peak energy. Our simulations were performed with a photon to electron ratio N γ /N e = 10 5 , as determined by observations of the GRB prompt emission. We searched an exhaustive parameter space to determine if the photospheric process can match the observed high-energy spectrum of the prompt emission. If we do not consider electron re-heating, we determined that the best conditions to produce the observed high-energy spectrum are low photon temperatures and high optical depths. However, for these simulations, the spectrum peaks at an energy below 300 keV by a factor ∼ 10. For the cases we consider with higher photon temperatures and lower optical depths, we demonstrate that additional energy in the electrons is required to produce a power-law spectrum above the peak-energy. By considering electron re-heating near the photosphere, the spectrum for these simulations have a peak-energy ∼ 300 keV and a power-law spectrum extending to at least 10 MeV with a spectral index consistent with the prompt emission observations. We also performed simulations for different values of N γ /N e and determined that the simulation results are very sensitive to N γ /N e . Lastly, in addition to Comptonizing a Blackbody spectrum, we also simulate the Comptonization of a f ν ∝ ν −1/2 fast cooled synchrotron spectrum. The spectrum for these simulations peaks at ∼ 10 4 keV, with a flat spectrum f ν ∝ ν 0 below the peak energy.

Monte Carlo Radiation Transfer Simulations of Photospheric Emission in Long-Duration Gamma-Ray Bursts

“…On the other hand, the photospheric emission has been shown to be able to reproduce all the correlations (Lazzati et al 2011(Lazzati et al , 2013López-Cámara et al 2014) but can hardly produce a spectrum that has prominent nonthermal tails at both low-and high-frequencies. While sub-photospheric dissipation can cure the high-frequency problem producing prominent non-thermal tails (Pe'er et al 2006;Giannios 2006;Giannios & Spruit 2007;Beloborodov 2010;Vurm et al 2011;Ito et al 2013Ito et al , 2014Chhotray & Lazzati 2015;Santana et al 2016), it is still unclear whether the lowfrequency tails can be reproduced in photon-starved conditions without invoking an external radiation mechanism such as synchrotron (Pe'er & Ryde 2011;Chhotray & Lazzati 2015). Encouraging results, however, have been obtained for photospheric emission with contributions from an subdominant synchrotron component (Vurm & Beloborodov 2015).…”

X-ray flares in GRBs: general considerations and photospheric origin

“…where Γ is the Lorentz factor of the matter producing the flare, F obs (νX ) is the observed specific flux at νX = 2keV during the flares starting at 300sec (Santana & Kumar 2015), dL is the luminosity distance and γe(νSA) is the typical thermal Lorentz factors of the electrons radiating synchrotron at νSA and is given by:…”

Diffusive photospheres in gamma-ray bursts

“…Most papers dealing with the photospheric emission, e.g. (Mészáros & Rees 2000;Pe'er 2008;Pe'er & Ryde 2011;Beloborodov 2011;Lundman et al 2013;Santana et al 2016;Bhattacharya et al 2018), for a review see Pe'er & Ryde (2017), adopt the hydrodynamic model of a steady and infinite wind. However, finite duration of GRBs implies finite width of the wind.…”