MAXI/GSC detection of an X-ray flare-like activity probably from the high mass X-ray binary EXO 1722-363

ATel #9412; H. Negoro, M. Nakajima (Nihon U.), M. Serino, T. Mihara (RIKEN), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara (JAXA), M. Sugizaki, W. Iwakiri, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), N. Kawai, 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 26 Aug 2016; 08:34 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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At 05:06 UT on 2016 August 23, the MAXI/GSC nova-alert system triggered faint X-ray enhancement from the region positionally consistent with the super-giant high mass X-ray binary pulsar EXO 1722-363 (a.k.a., X 1722-36, IGR J17252-3616).

The enhancement has been recognized in at least 9 scanning observations from 3:34 on August 23 to 15:55 on the same day. No observations were made after 18:19 on Aug. 22 by 03:34 on Aug. 23, and after 17:27 on Aug. 23 by 02:43 on Aug. 24. The center position of the enhancement is

(R.A., Dec) = (261.30 deg, -36.58 deg) = (17 25, -36 34) (J2000)

with an statistical and additional systematic uncertainty of 0.6 deg (90% containment radius).
The average 2-10 keV X-ray flux in the above 9 observations on Aug. 23 was (6.1+/-1.0)e-10 erg/cm^2/s (~30 mCrab), and the peak flux was about 90 mCrab at 06:39 Aug. 23.
The 413-sec X-ray pulsar EXO 1722-363 is 0.30 deg from the above center position and inside of the error circle. Since 2009, MAXI/GSC has observed the EXO 1722-363 region at a flux level of about 3 mCrab (or less) including the Galactic ridge X-ray emission, which is consistent with previous observations (Takeuchi et al. 1990,PASJ, 42, 287; Corbet et al. 2005, ApJ, 633, 377; Manousakis & Walter 2011, A&A, 526, A62). EXO 1722-363, however, also exhibited a flaring activity reaching about 80 c/s/PCU in the 2-30 keV band in 1998 (see Fig.2 of Corbet et al. 2005), which is comparable to this activity. With orbital parameters given by Manousakis & Walter 2011, we obtain an orbital phase on August 23 of 0.35, which is out of the eclipse.

We cannot exclude possibilities of other sources than EXO 1722-363 because of a source crowed region near the Galactic center (l= 351.3, b=-0.5). Then, we also name the source MAXI J1725-365.

MAXI public data archive for EXO 1722-363