TITLE: GCN CIRCULAR NUMBER: 13895 SUBJECT: GRB 121025A: MAXI/GSC detection DATE: 12/10/25 10:50:20 GMT FROM: Mikio Morii at Tokyo Inst Tech M. Asada, H. Negoro (Nihon U.), M. Serino (RIKEN), M. Morii (Tokyo Tech), Nakahira (JAXA), K. Yamaoka (Waseda U.), S. Ueno, H. Tomida, M. Ishikawa (JAXA), T. Mihara, M. Sugizaki, T. Yamamoto, J. Sugimoto, M. Matsuoka (RIKEN), N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), A. Yoshida (AGU), H. Tsunemi, M. Kimura (Osaka U.), M. Nakajima (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.) M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.) report on behalf of the MAXI team: At 2012-10-25T07:46:30 UT, the MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source. The transient emission lasted at least 20 seconds within the 44 second long triangular transit response of MAXI/GSC. We identify this event as GRB 121025A. Assuming that the source flux was constant over the transit, we obtain the source position at (R.A., Dec) = (248.75 deg, +27.73 deg) = (16 35 00, +27 44 05)(J2000) with a 90% C.L. statistical error of 0.27 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). The 4-10 keV flux was 129 (+26/-22) mCrab. Within the scan transit, the variability of the source was marginally detected with a chance probability of 0.084 for 4 - 10 keV band. There was no significant excess flux in the previous transit at 06:13 UT and in the next transit at 09:18 UT with an upper limit of 20 mCrab for each. Follow-up observations are encouraged.