[vsnet-alert 15735] MAXI/GSC detection of an X-ray enhancement possibly from a variable star HD 347929/1RXS J180724.2+194217 (ATEL)

[Taichi Kato]

   This object is identical with ASAS 180724+1942.4, a supposed
RS CVn-type object (it has an interesting ASAS light curve!).

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ATEL #5075							     ATEL #5075

Title:	MAXI/GSC detection of an X-ray enhancement possibly from a
		variable star HD 347929/1RXS J180724.2+194217
Author:	H. Negoro, T. Onodera (Nihon U),  Y. Ogawa (Miyazaki U.),
		M. Serino (RIKEN),  S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M.
		Ishikawa, Y. Nakagawa (JAXA),  T. Mihara, M. Sugizaki, M. Morii, T.
		Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),  N. Kawai,
		R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech),  A. Yoshida, T. Sakamoto,
		Y. Nakano, Y. Kawakubo (AGU),  H. Tsunemi, M. Sasaki (Osaka U.), 
		M. Nakajima, K. Fukushima, K. Suzuki (Nihon U.),  Y. Ueda, M. Shidatsu,
		T. Kawamuro (Kyoto U.),  Y. Tsuboi, M. Higa (Chuo U.),  M. Yamauchi,
		K. Yoshidome, H. Yamada (Miyazaki U.),  K. Yamaoka (Nagoya U.) report
		on behalf of the MAXI team
Queries:	negoro at phys.cst.nihon-u.ac.jp
Posted:	20 May 2013; 14:00 UT
Subjects:X-ray, Star, Transient, Variables

On May 20, 2013, MAXI/GSC detected an X-ray enhancement from the position
at  (R.A., Dec.) = (271.75 deg, 19.65 deg) = (18 07  0, +19 38 60) (J2000)
with a 90% C.L. statistical error of 0.3 deg and an additional systematic
uncertainty of 0.1 deg (90% containment radius), if a constant source flux
is assumed.

The enhanced emission has been recognized since the scan transit at 08:52
on May 19, 2013 (UT).  At previous scan transits, the region was in an
undetectable area of the detectors for about 1 day. Thus, the start time
of this event is uncertain, but a linear extrapolation of the observed
gradual increase in X-ray flux suggests that its started around the beginning
of May 19.

The average 4-10 keV flux during eight scan transits from 11:58 to 22:48
on May 19 was 30+- 5 mCrab, and that of the following three scan transits
up to 03:26 on May 20 was 22+-7 mCrab. 

The source position is consistent with that of a variable star  HD 347929
(1RXS J180724.2+194217), apart from the best position by 6.6 arcmin, and
a similar X-ray flux was observed from the source with MAXI/GSC on June
27, 2010 (Usui et al. ATel #2700, also see Keanna et al. ATel #2701). However,
a possibility of a new X-ray transient cannot be excluded. We tentatively
name the source MAXI J1807+196.

Followup observations are encouraged.