TITLE: GCN CIRCULAR NUMBER: 32756 SUBJECT: MAXI/GSC refined analysis of the bright X-ray afterglow of GRB 221009A/Swift J1913.1+1946 DATE: 22/10/14 14:29:30 GMT FROM: Hitoshi Negoro at Nihon U K. Kobayashi, H. Negoro, M. Nakajima, M. Tanaka, Y. Soejima (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, J. Kohara (Chuo U.), M. Shidatsu, M. Iwasaki (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, T. Kurihara (JAXA), Y. Ueda, S. Ogawa, K. Setoguchi, T. Yoshitake, K. Inaba (Kyoto U.), M. Yamauchi, T. Sato, R. Hatsuda, R. Fukuoka, Y. Hagiwara, Y. Umeki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), and M. Sugizaki (NAOC) We reported the MAXI/GSC detection of the bright X-ray emission from GRB 221009A/Swift J1913.1+1946 (GCN 32632/ATel #15650) at 13:58 (T0+2.5 ks, where T0 is the Fermi GBM trigger time, GCN 32642), 15:31 (T0+8.0 ks), and 17:04 (T+13.6 ks) (ATel #15651). Fitting GSC energy spectra at these scans with an absorbed power-law model gives a photon index of 1.93 +/- 0.09, 2.1 +/- 0.3, and 2.1 +/- 0.5, respectively. All the errors quoted are at the 90% confidence level. Here we fixed the column densities at 6.75e21 cm^-2 at z = 0.151 and 5.4e21 cm^-2 at z = 0 obtained with the Swift XRT (GCN 32651). Unabsorbed 2-10 keV fluxes are (6.1 +/- 0.3)e-8 erg cm^-2 s^-1, (1.1 +/- 0.1)e-8 erg cm^-2 s^-1, and (0.7 +/- 0.1)e-8 erg cm^-2 s^-1, respectively. We also note that the source was not detected in the transit at 12:25 UT on October 9 (T0-3.1 ks) with an upper limit of 20 mCrab. We crosspost this report to the ATel and the GCN because we initially regarded this event as a galactic transient, and posted the preliminary results only to the ATel.