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MAXI/GSC detection of weak X-ray enhancement from the direction of NGC 6440

ATel #10821; H. Negoro (Nihon U.), S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA), T. Mihara, M. Sugizaki, S. Nakahira, W. Iwakiri, M. Shidatsu, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, K. Morita (Tokyo Tech), A. Yoshida, T. Sakamoto, M. Serino, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki, H. Kawai (Chuo U.), M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), T. Kawamuro (NAOJ), and K. Yamaoka (Nagoya U.)
on 5 Oct 2017; 15:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

Subjects: X-ray, Transient

Referred to by ATel #: 10826, 10827, 10832, 10843, 10891

MAXI/GSC detects weak X-ray enhancement from the direction of the globular cluster NGC 6440. The 4-10 keV X-ray flux has been increasing since around 2017 September 29. Using data in 7 scan transits from 23:08 on October 3 to 06:07 on October 4, we obtain the source position at
(R.A., Dec) = (267.335 deg, -20.047 deg) = (17 49 20, -20 02 49) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.38 deg and 0.28 deg, respectively. The roll angle of long axis from the north direction is 75.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scans was 25 +- 5 mCrab (4.0-10.0keV, 1 sigma error).

NGC 6440 is 0.33 deg apart, and marginally within the error region. At least 24 X-ray sources are present in the cluster (Pooley et al. 2002, ApJ, 573, 184), but the possibility of the new transient in or out of the cluster can not be excluded. Then, we tentatively name the source MAXI J1749-200.

Two accreting millisecond X-ray pulsars in NGC 6440 exhibited X-ray outbursts or enhancement with similar X-ray fluxes to this one. One is the 442 Hz pulsar SAX J1748.8-2021 (e.g., Altamirano et al. 2008, ApJ, 674, 45), which showed weak or bright outbursts for instance, in 2009 (ATel #2139, #2180 #2360) and in 2015 (ATel #7098). The other is the 206 Hz pulsar NGC 6440 X-2, showing weak outbursts in 2009 (ATel #2182; Heike et al. 2010, ApJ, 714, 894) and in 2010 (ATel #2500).
Followup observations of this transient are encouraged to identify this enhancement, and clarify the nature of the source.

MAXI J1749-200 or AMXP in NGC 6440