GRB spectrum from gradual dissipation in a magnetized outflow

Abstract

Modeling of many GRB prompt emission spectra sometimes requires a (quasi) thermal spectral component in addition to the Band function that sometimes leads to a double-hump spectrum, the origin of which remains unclear. In photospheric emission models, a prominent thermal component broadened by sub-photospheric dissipation is expected to be released at the photospheric radius, rph ∼ 1012 cm. We consider an ultra-relativistic strongly magnetized steady outflow with a striped-wind magnetic-field structure undergoing gradual and continuous magnetic energy dissipation at r < rs that heats and accelerates the flow to a bulk Lorentz factor Γ(r) = Γ∞min [1, (r/rs)1/3], where typically rph < rs. 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) distributed heating of all electrons (and e±-pairs), e.g. due to magneto-hydrodynamic instabilities. Steady-state 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 consistently peaks at (1 + z)Epk ∼ 0.2 − 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, and (iv) the ∼0.5(1 + z)−1 keV X-ray emission is suppressed in scenario (a) whereas it is expected in scenario (b). Energy-dependent linear polarization can differentiate between the two particle heating scenarios.

A marginally fast-cooling proton-synchrotron model for prompt GRBs

“…In this scenario, the injection rate of electrons on the synchrotron radiation spectrum increases rapidly with time leading to the hardening of the corresponding radiation spectrum (Liu et al 2020). Gill et al (2020) investigated the steady-state photon spectra produced by electrons that are either accelerated via magnetic reconnection into a power law or heated via magnetohydrodynamic instabilities in a strongly magnetized relativistic outflow. In their scenario, power-law electrons cool mainly by synchrotron radiation, while the almost monoenergetic electrons from heating cool by Comptonization on thermal peak photons.…”

Electromagnetic counterparts of compact binary mergers

“…If the acceleration time scale becomes comparable with the electrons' cooling time, then the synchrotron cooling frequency can be kept close to the peak energy, since particles are not allowed to cool down. This scenario was proposed in different dissipation models as a way to explain the observed spectral breaks in the X-rays (Beniamini et al 2018;Gill, Granot & Beniamini 2020). The reacceleration of particles would require the presence of small-scale turbulence and/or the close by magnetic reconnection islands.…”

GRB Polarization: A Unique Probe of GRB Physics

“…The final outcome depends on the nature of the dissipation and how that leads to particle acceleration/heating. Gill et al [128], who carried out numerical simulations, and Beniamini and Giannios [129], who performed semi-analytic calculations, considered a steady PFD striped wind outflow, which is heated due to magnetic dissipation commencing at radii when the flow is optically thick to Thomson scattering with initial τ T0 = 100. At higher τ T , and equivalently lower radii, the flow maintains thermal equilibrium while it is being accelerated due to gradual magnetic dissipation.…”

“…The observed spectrum is effectively a sum over the optically thin spectra. See [128] for more details.…”