MAXI/GSC detection of a new X-ray outburst from the Be/X-ray binary pulsar XTE J1858+034

ATel #17260; H. Nishio, M. Nakajima, H. Negoro, K. Takagi, H. Takahashi, K. Tatano (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, Y. Yatsu (Science Tokyo), 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.), S. Yamada (Tohoku U)
on 3 Jul 2025; 04:28 UT
Credential Certification: Motoki Nakajima (nakajima.motoki@nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered an X-ray transient source on 2025-07-01. Assuming that the source flux was constant over the transits between 2025-07-01T00:00 and 2025-07-02T06:55, we obtain the source position to be:
(R.A., Dec) = (284.494 deg, 3.741 deg) = (18 57 58, +03 44 27) (J2000),
with a statistical 90% C.L. elliptical error region with long and short radii of 0.37 deg and 0.3 deg, respectively. The roll angle of long axis from the north direction is 180.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 19 +- 4 mCrab (4.0-10.0keV, 1 sigma error). The error region includes the position of the Be/X-ray binary pulsar XTE J1858+034. The source light curve suggests that the current X-ray outburst began on 2025-06-24 (MJD 60850). The latest day-averaged flux in the 2–20 keV band, observed on 2025-07-02, is approximately 15 mCrab. This marks the sixth outburst from XTE J1858+034 since its discovery in 1998 (IAUC 6826 : 2). The previous outburst was observed on 2023-03-30 (ATel #15970). Although no significant brightening has yet been observed in the Swift/BAT hard X-ray monitoring data, we note that during the 2019 outburst, the hard X-ray flux exhibited a delay of several days relative to the soft X-ray band. Therefore, confirmation of the current outburst by other instruments is anticipated within a few days.