MAXI/GSC detection of an X-ray outburst from the millisecond X-ray pulsar MAXI J1957+032

ATel #17170; H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), Y. Tsuboi, H. Sugai, N. Nagashima (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, 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.), and S. Yamada (Tohoku U)
on 7 May 2025; 09:12 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on a soft X-ray transient source at 18:18 UT on May 6, 2025. Assuming that the source flux was constant over four scan transits from 16:45 to 22:57 on May 6, we obtain the source position at
(R.A., Dec) = (299.210 deg, 3.841 deg) = (19 56 50, +03 50 27) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.51 deg and 0.46 deg, respectively. The roll angle of the long axis from the north direction is 141.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 18 +- 5 mCrab (4.0-10.0keV, 1 sigma error).

The position is consistent with that of the millisecond X-ray pulsar MAXI J1957+032, which was independently discovered in 2015 by MAXI (Negoro et al. ATel #7504) and INTEGRAL (Cherepashchuk et al. ATel #7506). After that, MAXI J1957+032 exhibited short outbursts lasting several days at least five times (before this outburst. e.g. Beri et al. 2019, MNRAS 486, 1620; Negoro et al. ATel #15440), and NICER revealed that the source was a millisecond X-ray pulsar in 2022 outburst (Sanna et al. 2022, MNRAS 516, L76).

We just noticed that the EP-WXT team also reported their detection of MAXI J1957+032 in the GCN Circular (Sun et al. GCN Circ. #40375).

MAXI data for MAXI J1957+032