Very High Lorentz Factor Fireballs and Gamma-Ray Burst Spectra

TL;DR

This paper proposes a model where collisionless bulk acceleration in GRB fireballs leads to very high Lorentz factors, explaining observed spectral features, delays, and potential neutrino signals, advancing understanding of GRB emission mechanisms.

Contribution

It introduces the concept of collisionless bulk acceleration beyond the photosphere, resulting in very high Lorentz factors and explaining various observed GRB spectral and temporal features.

Findings

High Lorentz factors (Γ ~ 10^3-10^6) can be achieved via collisionless bulk acceleration.

Synchrotron emission from internal shocks explains GeV spectral components and delays.

Predicted anticorrelation between TeV neutrinos and GeV gamma-rays could be detectable with IceCube.

Abstract

Collisionless entrainment of the surrounding matter imports the relativistic baryon component in the Gamma-Ray Burst (GRB) fireball frame. We show that half the fireball energy can be transferred from radiation to the comoving hot motions of baryons under the photosphere. The yet baryon-poor fireball can reexpand to a very high Lorentz factor (VHLF) \Gamma ~ 10^3-10^6 by its own relativistic collisionless pressure beyond the photosphere (so-called collisionless bulk acceleration), leading to internal and external shocks. A simple synchrotron emission from the VHLF internal shocks produces (i) the extra power-law spectral component with variability observed in the Fermi GeV bursts, up to the TeV range for the future Cherenkov Telescope Array (CTA), (ii) the GeV onset delay with a weak luminosity dependence t_{delay} ~ L^{-1/5}, and (iii) the spectral break of GRB 090926 by the synchrotron cooling break or the maximum synchrotron cutoff limited by the dynamical time, not by the e+- creation cutoff. The relativistic baryon component could also heat the photospheric thermal photons into the main GRB Band spectrum via pp, p\gamma (Bethe-Heitler and photomeson), and Coulomb thermalization processes. In this hot photosphere-internal-external shock model, we can predict the anticorrelation of ~TeV neutrinos and GeV gamma-rays, which may be detectable using IceCube. The spectral peak and luminosity (Yonetoku) relation is also reproduced if the progenitor stars are nearly identical. We also discuss the steep/shallow decay of early X-ray afterglows and short GRBs.