X-ray Transient detected by MAXI/GSC

ATel #2476; H. Negoro (Nihon U.), M. Morii, K. Sugimori, N. Kawai (Tokyo Tech), T. Matsumura, Yamazaki (Chuo U.), M. Matsuoka, K. Kawasaki, S. Ueno, H. Tomida, M. Suzuki, M. Ishikawa (JAXA), T. Mihara, M. Sugizaki, M. Kohama, Y. E. Nakagawa, T. Yamamoto, T. Saotome (RIKEN), A. Yoshida, K. Yamaoka, S. Nakahira (AGU), H. Tsunemi, M. Kimura (Osaka U.), M. Nakajima, S. Miyoshi, H. Ozawa, R. Ishiwata (Nihon U.), Y. Ueda, N. Isobe, S. Eguchi, K. Hiroi (Kyoto U.), A. Daikyuji (Miyazaki U.), A. Uzawa (Chuo U.) on behalf of the MAXI team
on 12 Mar 2010; 18:51 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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MAXI/GSC detected an X-ray transient at the scan transit centered at UT 2010-03-11T02:42:38. The averaged source flux during the scan is about 180 mCrab in the 4-10 keV band. Enhanced emission with no significant time variation continued during 45 seconds transit time. There was no significant detection at this location in the previous and following orbits with an upper limit of 20 mCrab, indicating the duration time of this event is less than 92 min.
Assuming that the source flux was constant over the transit, the nominal location of the source is determined as

(R.A., Dec)=(261.38 deg, -32.85 deg) = (17:25:31,-32:51:00) (J2000)

with a rectangular 90% statistical error box with the following corners:

(R.A., Dec) = (+261.73, -32.31) = (17:26:56, -32:18:26)
(R.A., Dec) = (+261.38, -32.21) = (17:25:32, -32:12:46)
(R.A., Dec) = (+260.99, -33.23) = (17:23:58, -33:13:47)
(R.A., Dec) = (+261.35, -33.32) = (17:25:23, -33:19:28)

The systematic uncertainties are calibrated using bright sources found in the same scan, and therefore, are much smaller.
This location is consistent with source positions of the short X-ray transient detected with MAXI/GSC on 2010-02-02 (ATel #2415), and the X-ray burster, IGR J17254-3257 (= 1RXS J172525.5-325717). The limited spatial resolution of MAXI does not allow us to identify this transient source as these sources. Note, however, that IGR J17254-3257 exhibited two types of X-ray bursts lasting ~10 second and several hundred seconds (Chenevez et al. 2007). The RXTE Galactic Center observation (http://lheawww.gsfc.nasa.gov/users/craigm/galscan) shows that the flux of IGR J17254-3257 has been about 6-8 mCrab since the beginning of last February, suggesting that the source has been in an active phase.
Preliminary analysis shows that an apparent energy spectrum obtained with MAXI/GSC is softer than that of the short burst on Feb. 2. Further information on this events will be posted at the MAXI web site http://maxi.riken.jp at the news section.

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