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Recent short-term X-ray activity of MAXI J1957+032

ATel #8529; K. Tanaka, H. Negoro (Nihon U.), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Serino, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, M. Arimoto, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, R. Imatani (Osaka U.), M. Nakajima, T. Masumitsu (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, S. Kanetou, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, D. Itoh, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.), M. Morii (ISM) report on behalf of the MAXI team
on 11 Jan 2016; 05:21 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Black Hole, Neutron Star, Star, Transient, Variables

We report recent short-term X-ray activity of MAXI J1957+032 (IGR J19566+0326) discovered in 2015 May (ATel. #7504, #7506). At 08:27 UT on 2016 January 7 (MJD 57394.352), the MAXI nova-alert system triggered faint X-ray emission from the location consistent with the position of MAXI J1957+032. The sun angle constraint did not allow us to perform Swift/XRT follow-up observation.

Almost stable enhanced X-ray emission of approximately 20-30 mCrab was detected on January 7 and 8 with MAXI/GSC. 2-20 keV (2-4 keV, 4-10 kev, 10-20 keV) photon fluxes (ph/cm2/s) are shown below,

Jan. 6: 0.014 +/- 0.011 (-0.002 +/- 0.005, 0.002 +/- 0.006, 0.013 +/0.08)
Jan. 7: 0.662 +/- 0.013 ( 0.033 +/- 0.006, 0.027 +/- 0.006, 0.004 +/0.08)
Jan. 8: 0.712 +/- 0.013 ( 0.011 +/- 0.006, 0.033 +/- 0.006, 0.020 +/0.09)
Jan. 9: 0.001 +/- 0.014 ( 0.005 +/- 0.006, 0.012 +/- 0.007, -0.016 +/0.08).

The nature of the source is still unknown. This is the 3rd short-term activity from the source. All the activities lasted less than a few days, and the peak fluxes were similar (10-30 mCrab). The last activity was observed on 2015 October 06 (ATel #8143, #8146, #8149, #8197), and no apparent long-term periodic activity was recognized.

MAXI public data for MAXI J1957+032