MAXI/GSC detection of a new X-ray brightening from Swift J0243.6+6124, LS I +61 303, or a new X-ray transient MAXI J0242+611

ATel #15983; K. Setoguchi (Kyoto U.), H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima (Nihon U.), T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), M. Shidatsu, M. Iwasaki (Ehime U.), N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, T. Kurihara (JAXA), Y. Ueda, S. Ogawa, T. Yoshitake, K. Inaba, Y. Nakatani (Kyoto U.), M. Yamauchi, T. Sato, R. Hatsuda, R. Fukuoka, Y. Hagiwara, Y. Umeki, Y. Otsuki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC), and W. Iwakiri (Chiba U.)
on 9 Apr 2023; 05:09 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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The MAXI/GSC nova alert system triggered on a bright X-ray transient source at 13:23 UT on April 8, 2023. Assuming that the source flux was constant over six transits from 04:06 to 19:35 on April 8, we obtain the source position at
(R.A., Dec) = (40.585 deg, 61.145 deg) = (02 42 20, +61 08 43) (J2000)
with a 90% C.L. statistical error of 0.58 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 23 +- 7 mCrab (4.0-10.0keV, 1 sigma error).

On the basis of the fitting results, this position is consistent with those of Swift J0243.6+6124 and LS I +61 303. Since both sources are transient Be X-ray pulsars, we suggest that this transient is Swift J0243.6+6124 or LS I +61 303. The BAT transient monitor data (https://swift.gsfc.nasa.gov/results/transients/), however, do not show any flux increase in both data. Therefore, we also tentatively name the source MAXI J0242+611.

Followup observations are encouraged.

Trigger Information