MAXI/GSC detection of X-ray emission probably from the black hole candidate XTE J2012+381

ATel #15826; T. Kawamuro (RIKEN), H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima (Nihon U.), T. Mihara, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU), Y. Tsuboi, J. Kohara, S. Urabe, S. Nawa, N. Nemoto (Chuo U.), W. Iwakiri (Chiba 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, K. Setoguchi, T. Yoshitake, K. Inaba (Kyoto U.), M. Yamauchi, T. Sato, R. Hatsuda, R. Fukuoka, Y. Hagiwara, Y. Umeki (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
on 26 Dec 2022; 05:05 UT
Credential Certification: Taiki Kawamuro (kawamuro@kusastro.kyoto-u.ac.jp)

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The MAXI/GSC nova alert system triggered a bright X-ray transient source spatially consistent with XTE J2012+381 at a 90% confidence level at UT 21:27 on 25, December. Assuming that the source flux was constant over transits during almost one day since the trigger, we estimated an averaged X-ray flux to be 39 +- 8 mCrab (4.0--10.0keV, 1 sigma error), and seemingly the source is bright rather in the 2--4 keV. XTE J2012+381 is a black hole candidate according to past X-ray studies using RXTE and ASCA, which found X-ray spectra typical for the candidate class; they were well reproduced with disk-black-body plus hard power-law components (e.g., White et al. 1998; Vasiliev, Trudolyubov, and Revnivtsev 2000). Historically, on May 27, 1998, the object was discovered as a transient source by All Sky Monitor of RXTE, and the monitor showed that after reaching a flux level as high as ~ 110 mCrab in the 2--10 keV band around June, 1998 (Marshall et al. 1998), it gradually decayed. Since these past observational works around 1998 (e.g., Campana et a. 2002), little attention has been paid to the object due to it being in a quiescent state. Thus, the current brightening phase should be providing the second opportunity for the better understanding of the nature of XTE J2012+381, and multi-wavelength follow-up observations are encouraged.