{
  "body": "M. Uenishi (Ehime U.), H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), \nT. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), \nT. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), \nY. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), \nM. Shidatsu, C. Kang, T. Nakamoto, T. Usuki, S. Yatsuzuka (Ehime U.),\nI. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), \nY. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), \nK. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), \nand S. Yamada (Tohoku U)\n\nThe MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source \nat 10:37:16 UT on December 20, 2025.\nAssuming that the source flux was constant over the transit,\nwe obtain the source position at\n(R.A., Dec) = (19.609 deg, -15.607 deg) = (01 18 26, -15 36 25) (J2000) \nwith a statistical 90% C.L. elliptical error region \nwith long and short radii of 0.38 deg and 0.28 deg, respectively. \nThe roll angle of long axis from the north direction is 173.0 deg counterclockwise. \nThere is an additional systematic uncertainty of 0.1 deg (90% containment radius).\nThe X-ray flux averaged over the scan was 104 +- 23 mCrab\n(4.0-10.0keV, 1 sigma error).\nWithout assumptions on the source constancy, we obtain a rectangular error box\nfor the transient source with the following corners:\n(R.A., Dec) = (18.401, -15.400) deg = (01 13 36, -15 24 00) (J2000) \n(R.A., Dec) = (18.492, -16.164) deg = (01 13 58, -16 09 50) (J2000) \n(R.A., Dec) = (21.034, -15.868) deg = (01 24 08, -15 52 04) (J2000) \n(R.A., Dec) = (20.934, -15.105) deg = (01 23 44, -15 06 18) (J2000)\n\nThere was no significant excess flux in the previous transit at 09:04 UT \nand in the next transit at 12:10 UT with an upper limit of 20 mCrab for each.",
  "createdOn": 1766233065603,
  "format": "text/plain",
  "eventId": "GRB 251220A",
  "circularId": 43156,
  "bibcode": "2025GCN.43156....1U",
  "subject": "GRB 251220A: MAXI/GSC detection",
  "submittedHow": "web",
  "submitter": "Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi@nihon-u.ac.jp>"
}