The spectrum of γ-ray burst: a clue

Abstract

In this work, we numerically calculate the thermal radiation efficiency of the baryonic outflow. The possible outflow acceleration in the transparent stage, which lowers thermal radiation efficiency, has been taken into account. In the standard internal shock model for the prompt emission, the fast shells should move with a typical Lorentz factor ∼5 Γi otherwise the γ‐ray burst (GRB) efficiency will be in disagreement with the observations, where Γi is the bulk Lorentz factor of the shocked/emitting region. The photosphere radius of these fast shells is small and the thermal radiation is too strong to be effectively outshone by the internal shock emission. This is particularly the case for some extremely bright events having Γi∼ 103, like GRBs 080319B and 080916C. The absence of a distinct thermal component in the spectrum of most GRBs challenges the standard internal shock model and may suggest a non‐baryonic (magnetic) outflow component. Though the magnetic outflow model seems favoured by more and more data, it can hardly reproduce the typical GRB spectrum. In the photosphere‐gradual magnetic dissipation scenario, the spectrum cuts off at ∼1 GeV, too low to account for the observations of GRBs 080916C. In the sudden magnetic energy dissipation model, the low‐energy spectrum is expected to be Fν∝ν−1/2, too soft to be consistent with the data Fν∝ν0. We speculate that the low‐energy spectrum puzzle could be unveiled by the mechanism that particles, in the magnetic dissipation process, are repeatedly accelerated.

PHOTOSPHERE EMISSION FROM A HYBRID RELATIVISTIC OUTFLOW WITH ARBITRARY DIMENSIONLESS ENTROPY AND MAGNETIZATION IN GRBs

“…When σ 0 ≪ 1 (panels (a-c), the fireball case), the photosphere emission is bright, and one would expect a bright thermal component sticking out from the Band component. This is consistent with the results of Fan (2010). In particular, when η is large enough, the spectrum is completely dominated by the thermal component, similar to the case of GRB 090902B 4 .…”

Is GeV emission from Gamma-Ray Bursts of external shock origin?

“…Beloborodov 2010;Lazzati & Begelman 2010). The lack of photosphere emission then demands a Poynting flux dominated outflow at least for this burst (Zhang & Pe'er 2009;Fan 2010). This argument, along with other evidence uncovered in other GRBs (e.g.…”

Open questions in GRB physics

“…If the entire non-thermal spectrum is from the internal shocks, then the photosphere emission of the hot fireball is expected to be bright enough to show up above the detected Band spectrum [153,154]. This led to the suggestion that GRB 080916C and most LAT GRBs have σ ≫ 1 at the central engine, and probably also in the emission region as well [153].…”

“…If the observed Band component is the non-thermal emission from internal shocks, one would expect a bright quasi-thermal spectral component from the fireball photosphere which outshines the non-thermal component, making the observed spectrum significantly deviated from the simple Band form. This led to the suggestion that the ejecta has to be Poynting flux dominated in order to suppress the bright photosphere thermal emission [153,154]. Since most Fermi LAT GRBs have Band-only spectra similar to GRB 080916C [155], one may speculate that most GRBs may function similar to GRB 080916C.…”