Fermi Gamma-ray Space Telescope

How far from the central engine are the GRBs produced?

Z. Lucas Uhm
(Z. Lucas Uhm, Donggeun Tak, Bing Zhang, Judith Racusin, Daniel Kocevski, Sylvain Guiriec, and Julie McEnery)


The physical mechanism of gamma-ray bursts (GRBs) remains elusive even after many decades since the discovery in late 1960s. 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 under debate predict different characteristic distances from the engine. Here we present a simple but convincing study addressing this question, by making use of the so-called "high-latitude emission" (HLE). The signature of HLE has never been clearly identified previously in the prompt phase of GRBs. We show that our detailed numerical modeling predicts a "smoking-gun" signature of HLE in the decaying phase of "broad pulses" during the prompt emission. In search of this signature, we select a sample of bright broad-pulses of Fermi-GBM GRBs from the years 2008 - 2017 and find that at least a half of the sample exhibits the predicted HLE signature. We show that the gamma-ray emitting region of those GRBs should be located at ~10^{16} cm from the central engine (or even beyond), which strongly disfavors the photosphere models and small-radii internal shock models but favors magnetic dissipation models with a large emission radius.