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

The monitor of the 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 had a peculiarly narrow spectral energy distribution, which can be represented by a Comptonized blackbody model. This spectrum can be interpreted as photospheric emission from a low baryon-load GRB fireball. Calculating the parameter of the fireball, we found the size of the base of the flow to be $r_0$$=$ (4.3 $\ \pm\ $ 0.9) $\times$ 10 $^{9}\ Y'^{-3/2}\ $ cm, the Lorentz factor of the plasma is $\Gamma$$=$ (110 $\ \pm\ $ 10) $\ Y'^{\ 1/4}$ , where $Y'$ is a ratio between the total fireball energy and the energy in the blackbody component of the gamma-ray emission. This $r_0$ is a factor of a few times 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.