Peculiarly Narrow SED of GRB 090926B with MAXI and Fermi/GBM

TL;DR

This paper reports the detection and analysis of an unusual gamma-ray burst with a peculiarly narrow spectral energy distribution, interpreted as photospheric emission, with derived fireball parameters indicating a larger initial size and lower Lorentz factor than typical GRBs.

Contribution

It presents the first detailed spectral analysis of GRB 090926B showing a narrow spectrum modeled by Comptonized blackbody, and derives unique fireball parameters indicating different physical conditions.

Findings

The spectrum is best fit by a Comptonized blackbody model.

The initial fireball size is larger than typical GRBs.

The Lorentz factor of the plasma is lower than in other blackbody-emitting GRBs.

Abstract

The monitor of all-sky X-ray image (MAXI) Gas Slit Camera (GSC) on the International Space Station (ISS) detected a gamma-ray burst (GRB) on 2009, September 26, GRB\,090926B. This GRB had extremely hard spectra in the X-ray energy range. Joint spectral fitting with the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope shows that this burst has peculiarly narrow spectral energy distribution and is represented by Comptonized blackbody model. This spectrum can be interpreted as photospheric emission from the low baryon-load GRB fireball. Calculating the parameter of fireball, we found the size of the base of the flow $r_0 = (4.3 \pm 0.9) \times 10^{9} \, Y^{\prime \, -3/2}$ cm and Lorentz factor of the plasma $\Gamma = (110 \pm 10) \, Y^{\prime \, 1/4}$, where $Y^{\prime}$ is a ratio between the total fireball energy and the energy in the blackbody component of the gamma-ray emission. This $r_0$ is factor of a few larger, and the Lorentz factor of 110 is smaller by also factor of a few than other bursts that have blackbody components in the spectra.