Thermal Radiation from Relativistic GRB Jets from Collapsars

Abstract

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the presence of relativistic jets, instabilities such as the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability create colli-sionless shocks, which are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The "jitter" radiation from deflected electrons in small-scale magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation, a case of diffusive synchrotron radiation, may be important to understand the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow

“…While all of these effects have the potential of softening the low-energy slope from purely thermal emission, simulations are needed in order to determine their relative importance. Indeed, recent simulations by Mizuta et al (2010) show thermal emission from a GRB jet with α = −0.5, much softer than a blackbody spectrum with α = 1.…”

“…This does not necessarily rule out a photospheric origin for the emission. In fact, there are many reasons to expect the photospheric emission to deviate from a simple Planck function, and several authors have recently considered models that give rise to photospheric emission with a broad, Band-like spectrum (Pe'er et al 2006;Beloborodov 2010;Lazzati & Begelman 2010;Mizuta et al 2010). A broadened photospheric component is also to be expected from the simple fact that the observed spectrum is most likely composed of emission arising from different regions in space.…”

GeV emission from short gamma-ray bursts: the case of GRB 081024B

“…We refer the reader interested in this alternative explanation to papers such as e.g. Ioka (2010), Mizuta et al (2010), Pe'er & Ryde (2010, Toma et al (2010), and references therein.…”

Can Gamma-Ray Burst Jets Break Out the First Stars?

“…Due to the additional collimation by the gas pressure, θ j becomes smaller as the jet propagates (e.g., Zhang et al 2003;Mizuta et al 2006Mizuta et al , 2010. The smaller θ j leads the larger L iso (= 2L j /θ 2 j ) so that our constant θ j is a conservative assumption for the jet breakout.…”