MAXI/GSC detection of a probable new soft X-ray transient MAXI J1857-086 or renewed activity of Swift J1858.6-0814

ATel #13455; H. Negoro, R. Takagi (Nihon U.), R. Adachi (Tokyo Tech), M. Nakajima, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), T. Mihara, C. Guo, Y. Zhou, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, H. Nishida, A. Yoshida (AGU), Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, M. Oeda, K. Shiraishi (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, R. Shimomukai, M. Tominaga (JAXA), Y. Ueda, A. Tanimoto, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake (Kyoto U.), H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), M. Yamauchi, S. Iwahori, Y. Kurihara, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 6 Feb 2020; 11:52 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 enhanced soft X-ray emission at 00:52 UT on 2020 February 6. Assuming that the source flux was constant over nine scan transits from 18:40 on February 5 to 08:37 on February 6, we obtain the source position at

(R.A., Dec) = (284.318 deg, -8.604 deg) = (18 57 16, -08 36 14) (J2000)

with a statistical 90% C.L. elliptical error region with long and short radii of 0.33 deg and 0.21 deg, respectively. The roll angle of the long axis from the north direction is 37.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 28 +- 5 mCrab (4.0-10.0keV, 1 sigma error).

The source flux peaked at about 50 mCrab near the trigger time. The transient black hole candidate Swift J1858.6-0814 discovered on 2018 October 31 (Krimm et al. ATel #12151) lies 0.50 arc-deg from the above position, and just out of the error region. The BAT light curve shows that the source flux has been decreasing for more than 50 days (https://swift.gsfc.nasa.gov/results/transients/weak/SWIFTJ1858.6-0814/). There is no known bright catalogued X-ray source except for Swift J1858.6-0814, then, we tentatively name the source MAXI J1857-086. If the source is Swift J1858.6-0814, this enhancement implies renewed activity or a soft state transition.

Followup observations are strongly encouraged to identify this new transient.

Trigger information about MAXI J1857-086 or Swift J1858.6-0814