MAXI/GSC detection of an X-ray flare from II Peg on 2025-04-15 UT

ATel #17150; N. Nagashima, Y. Tsuboi (Chuo U.), H. Negoro (Nihon U.), M. Serino (AGU), T. Mihara (RIKEN), M. Nakajima, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, S. Sasao, A. Yoshida (AGU), H. Sugai (Chuo U.), M. Shidatsu, Y. Niida, C. Kang, T. Nakamoto (Ehime U.), I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, Y. K. Fujiwara (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), S. Yamada (Tohoku U), report on behalf of the MAXI team:
on 17 Apr 2025; 13:26 UT
Credential Certification: Yohko Tsuboi (tsuboi@phys.chuo-u.ac.jp)

Tweet

In the accumulated image from 2025-04-15 20:22 UT to 2025-04-16 19:25 UT, the MAXI/GSC nova alert system triggered on a source at a position consistent with that of an RS CVn star II Peg.

The position is determined as (R.A., Dec) = (358.526 deg, 28.340 deg) = (23 54 06, +28 20 23) (J2000) with a statistical 90% C.L. elliptical error region with long and short radii of 0.37 deg and 0.29 deg, respectively. The roll angle of the long axis from the north direction is 8.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). This position is consistent with that of the RS CVn star II Peg.

The X-ray flux averaged over the accumulated span is -1 +- 2 mCrab (4.0-10.0keV, 1 sigma error), which corresponds to a luminosity of 8 × 10³¹ erg s^-1 s ^-1 for a distance of 40 pc. The flux increased from 2025-04-15 21h to 2025-04-16 03h UT, remained at the top level until 2025-04-16 18h, and then decayed with an e-folding time of 0.16 days. The total energy released during the flare is 1 × 10 ³⁶ erg.