Evidence of high latitude emission in the prompt phase of GRBs: How far from the central engine are the GRBs produced?

TL;DR

This study uses numerical modeling of high-latitude emission in GRBs to estimate the emission region at about 10^{16} cm from the central engine, challenging some existing models and supporting magnetic dissipation scenarios.

Contribution

It provides the first observational evidence of high-latitude emission signatures in GRB prompt phases, estimating emission distances and constraining physical models.

Findings

HLE produces a spectral break in GRB spectra.

HLE can dominate the spectral peak during decay phase.

Emission region is estimated at ~10^{16} cm from the central engine.

Abstract

The physical mechanism of gamma-ray bursts (GRBs) remains elusive. One of the difficulties in nailing down their physical mechanism comes from the fact that there has been no clear observational evidence on how far from the central engine the prompt gamma-rays of GRBs are emitted while the competing physical mechanisms predict different characteristic distances. Here we present a simple study addressing this question by making use of the "high-latitude emission" (HLE). We show that our detailed numerical modeling exhibits a clear signature of HLE in the decaying phase of "broad pulses" of GRBs. We show that the HLE can emerge as a prominent spectral break in $F_{\nu}$ spectra and dominate the peak of $\nu F_{\nu}$ spectra even while the "line-of-sight emission" (LoSE) is still ongoing, hence providing a new view of HLE emergence. We remark that this "HLE break" could be hidden in some broad pulses, depending on the proximity between the peak energies of the LoSE and the HLE. Also, we present three examples of Fermi-GBM GRBs with broad pulses that exhibit the HLE signature. We show that their gamma-ray emitting region should be located at $\sim 10^{16}$ cm from the central engine, which disfavors the photosphere models and small-radii internal shock models but favors magnetic dissipation models with a large emission radius.