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MAXI/GSC detection of a hard X-ray transient MAXI J0058-721 in the SMC

ATel #9348; H. Negoro, M. Nakajima (Nihon U.), N. Kawai (Tokyo Tech), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, Y. Sugawara (JAXA), T. Mihara, M. Sugizaki, M. Serino, W. Iwakiri, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Isobe, S. Sugita, T. Yoshii, Y. Tachibana, Y. Ono, T. Fujiwara, S. Harita, Y. Muraki (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, R. Shomura (Osaka U.), K. Tanaka, T. Masumitsu, T. Kawase (Nihon U.), Y. Ueda, T. Kawamuro, T. Hori, A. Tanimoto (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, K. Furuya (Miyazaki U.), K. Yamaoka (Nagoya U.), Y. E. Nakagawa (JAMSTEC)
on 9 Aug 2016; 01:14 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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

Referred to by ATel #: 9362, 9677, 9731

The MAXI/GSC nova-alert system triggered on faint X-ray emission from the SMC at 13:17 UT on 2016 August 08 (MJD 57608.553). Using data of 18 scan transits from 17:14 on August 07 to 21:00 on August 08 and assuming a constant source flux over each transit, we obtained the position at

(R.A., Dec) = (14.511 deg, -72.165 deg) = (00 58 02, -72 09 52) (J2000)

with a 90% C.L. statistical error of 0.59 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux has increased since the beginning of August 07, and the average 4-10 keV flux during the above 18 scan transits is 9 +/- 3 mCrab.

We can not identify the source as any known X-ray source because of the relatively large error region for the source crowed SMC region and because of no such a bright source currently observed (e.g., Kennea et al. ATel #9299). Then we tentatively name the source MAXI J0058-721. The position is consistent with that of the Be X-ray pulsar SXP 202A currently in outburst (Coe et al. ATel #9307, Schmidtke et al. ATel #9320), which is about 14.7 arcmin from the above center position. The flux is, however, lower than that of MAXI J0058-721, by more than one order of magnitude. MAXI J0057-720 (Suzuki et al. ATel #5925, also see Kennea et al. #5930), which is positionally consistent with AX J0058-7203, is 8.3 arcmin from the center position. SXP 6.85 in an active phase (Kennea et al. ATel #9299) is just out of the 90% error region, and 41 arcmin apart.

A Swift/XRT ToO observation has been performed, and is expected to give a more precise position