GRB Spectrum from Gradual Dissipation in a Magnetized Outflow

TL;DR

This paper models gamma-ray burst spectra considering gradual magnetic dissipation in a magnetized outflow, revealing how different electron heating mechanisms influence spectral features and polarization, with implications for interpreting GRB observations.

Contribution

It introduces a detailed numerical model of GRB spectra incorporating magnetic dissipation and compares two electron heating scenarios, highlighting their spectral and polarization differences.

Findings

Thermal peak at 0.2-1 MeV, often subdominant in high energy injection cases.

Synchrotron cooling dominates for power-law electrons, Comptonization for distributed heating.

Low-energy break near thermal energy; polarization distinguishes heating mechanisms.

Abstract

Modeling of gamma-ray burst (GRB) prompt emission spectra sometimes requires a (quasi-) thermal spectral component in addition to the Band function. In photospheric emission models, a prominent thermal component broadened by sub-photospheric dissipation is expected to be released at the photospheric radius, $r_{\rm ph}\sim10^{12}\,$cm. We consider an ultra-relativistic strongly magnetized outflow with a striped-wind magnetic-field structure undergoing gradual and continuous magnetic energy dissipation at $r<r_s$ that heats and accelerates the flow, leading to a bulk Lorentz factor $\Gamma(r)=\Gamma_\infty\min[1,(r/r_s)^{1/3}]$, where typically $r_{\rm ph}<r_s$. Similar dynamics and energy dissipation rates are also expected in highly-variable magnetized outflows without stripes/field-reversals. Two modes of particle energy injection are considered: (a) power-law electrons, e.g. accelerated by magnetic reconnection, and (b) continuous distributed heating of all electrons (and $e^\pm$-pairs), e.g. due to MHD instabilities. Time-resolved energy spectra are obtained using a numerical code that evolves coupled kinetic equations for a photon-electron-positron plasma. We find that (i) the thermal component peaks at $(1+z)E_{\rm pk}\sim0.2-1\,$MeV, for a source at redshift $z$, and becomes subdominant if the total injected energy density exceeds the thermal one, (ii) power-law electrons cool mainly by synchrotron emission whereas mildly relativistic and almost monoenergetic electrons in the distributed heating scenario cool by Comptonization on thermal peak photons, (iii) both scenarios can yield a low-energy break at $E_{\rm br}\approx E_{\rm th}$, and (iv) the $0.5(1+z)^{-1}\,$keV X-ray emission is suppressed in the power-law injection case, but it is expected for the distributed heating scenario. Energy-dependent linear polarization can differentiate between the two energy injection cases.