GCN Circular 17860

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
Not found
 
 
).  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.

http://maxi.riken.jp/grbs/150518a/

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
Not found
 
 
), 
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
Not found
 
 
), 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.