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Short X-ray Transient detected by MAXI/GSC

ATEL # 2415; M. Suzuki (JAXA), H. Negoro (Nihon U.), M. Morii, K. Sugimori, N. Kawai (Tokyo Tech), M. Matsuoka, K. Kawasaki, S. Ueno, H. Tomida, M. Ishikawa (JAXA), T. Mihara, M. Kohama, Y.E. Nakagawa, M. Sugizaki, T. Yamamoto (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.) report on behalf of the MAXI team
on 3 Feb 2010; 17:02 UT
Password Certification: Motoko Suzuki (motoko@crab.riken.jp)

Subjects: X-ray, Transients

At the scan transit centered at UT 2010-02-02T17:14:52, MAXI/GSC detected a transient X-ray source, which was present at least for 15 seconds within the 110 seconds triangular transit response of MAXI/GSC. The nominal location of the source, assuming that the source flux was constant over the transit (which probably was not), is determined as

(R.A., Dec)=(261.05 deg, -32.94 deg) = (17 24 12, -32 56 24) (J2000)

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

(R.A., Dec)= (261.11 deg, -33.27 deg) = (17 24 26, -33 16 12)(J2000)
(R.A., Dec)= (260.75 deg, -33.23 deg) = (17 23 00, -33 13 48)(J2000)
(R.A., Dec)= (261.00 deg, -32.67 deg) = (17 24 00, -32 40 12)(J2000)
(R.A., Dec)= (261.36 deg, -32.72 deg) = (17 25 26, -32 43 12)(J2000)

The systematic uncertainties are calibrated using bright sources found in the same scan, and therefore, are much smaller.
If the source was only temporary bright during the scan, which is well expected for the case of a GRB/XRF or an X-ray burst, uncertainty along the scan direction becomes larger, as we cannot fit the light curve to the triangular transit response. Without assumptions on the source constancy, we obtain a rectangular error box with the following corners:

(R.A., Dec)= (260.33 deg, -34.11 deg) = (17 21 19, -34 06 36)(J2000)
(R.A., Dec)= (260.81 deg, -34.15 deg) = (17 23 14, -34 09 00)(J2000)
(R.A., Dec)= (261.89 deg, -31.74 deg) = (17 27 34, -31 44 24)(J2000)
(R.A., Dec)= (261.43 deg, -31.67 deg) = (17 25 43, -31 40 12)(J2000)

The nominal flux (4-10 keV) of the source was about 120 mCrab if the source was constant over the transit. The flux could be 300 mCrab or higher if the emission was present only for 15 seconds within the transit. There was no significant detection at this location in the previous and following orbits (92 min before or after the detection) with an upper limit of 20 mCrab. There is no known bright X-ray binary source at the detected position. At the galactic coordinates (l,b)=(354.1, 1.7), the source is likely to be Galactic, although extragalactic origin cannot be ruled out. With a very preliminary spectral analysis, the energy spectrum can be modeled by a highly absorbed (NH>1e23 cm-2) blackbody (kT~1.5 keV), suggestive of a type-1 X-ray burst at a large distance.
The scan light curve and the source location will be posted at the MAXI web site http://maxi.riken.jp at the news section.

Note: Since the nature of the source is unknown, this message is cross-posted to ATEL and GCN.


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