T. Sakamoto (AGU), M. Serino (RIKEN), S. Nakahira (JAXA), H. Negoro (Nihon U.),
S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA),
T. Mihara, M. Sugizaki, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),
N. Kawai, T. Yoshii, Y. Tachibana (Tokyo Tech),
A. Yoshida, Y. Kawakubo, H. Ohtsuki (AGU),
H. Tsunemi, R.Imatani (Osaka U.),
M. Nakajima, K. Tanaka, T. Masumitsu (Nihon U.),
Y. Ueda, T. Hori, T. Kawamuro (Kyoto U.),
Y. Tsuboi, S. Kanetou (Chuo U.),
M. Yamauchi, D. Itoh (Miyazaki U.),
K. Yamaoka (Nagoya U.), M. Morii (ISM)
We report further analysis using the MAXI/GSC data of MAXI GRB 150518A
(Kawamuro et al. GCN Circ. 17825). The MAXI GSC light curve shows a constant
flux during the scan. Due to a poor statistics, it is hard to claim a
variability in the light curve. The MAXI light curve plot is available from
the following URL.
The GSC spectrum (2-20 keV) using the data from 21:42:00 UT to 21:42:49 UT
is best fit by a simple power-law model. The best fit power-law photon index
is 1.3 +- 0.4. The column density is not well constrained from the GSC spectrum.
The unabsorbed flux in the 2-20 keV band is (1.0 +- 0.3) x 10^-8 erg/cm2/s.
From this best fit model, the unabsorbed X-ray flux in the 0.3-10 keV band
(Swift/XRT energy band) is (7.9 +- 2.4) x 10^-9 erg/cm2/s. No emission was
detected from the GRB position during the next scan starting from 23:11 UT at
the 1 sigma upper limit of ~9 mCrab.
Using the possible host galaxy redshift of 0.256 (Xu et al. GCN Circ. 17832),
the X-ray luminosity at the time of the MAXI detection is 1.7 x 10^48 erg/s (2-20 keV)
and 1.3 x 10^48 erg/s (0.3-10 keV). Assuming the onset of this GRB is the
detection by Konus-Wind (Golenetskii et al. GCN Circ. 17837), we found that
the MAXI luminosity around ~1000 sec after this onset is consistent with a typical
early afterglow emission seen by Swift/XRT (e.g., Racusin et al. 2011, ApJ, 738, 138).
On the other hand, if the possible emission seen by the Konus-Wind data around
the MAXI detection is real, the duration of GRB 150518A can be >1000 s. As we
reported above, the MAXI GSC spectrum is rather hard for an afterglow. In this
case, this GRB can be classified as an ultra-long GRB.
All the quoted errors are at the 90% confidence level. The assumed cosmological
parameters are H0 = 71 km s^-1 Mpc^-1, Omega_m = 0.27 and Omega_lamda = 0.73.