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MAXI/GSC detection of X-ray activities in the globular cluster M15

ATel #3356; M. Morii (Tokyo Tech), H. Negoro (Nihon U.), T. Mihara, M. Sugizaki, M. Serino, S. Nakahira, T. Yamamoto, T. Sootome, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), N. Kawai, K. Sugimori, R. Usui, T. Toizumi (Tokyo Tech), A. Yoshida, K. Yamaoka (AGU), H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), M. Nakajima, F. Suwa (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu (Kyoto U.), Y. Tsuboi, T. Matsumura, K. Yamazaki (Chuo U.) report on behalf of the MAXI team
on 17 May 2011; 17:11 UT
Credential Certification: Mikio Morii (morii@hp.phys.titech.ac.jp)

Subjects: X-ray, Binary, Globular Cluster, Transient, Variables

Referred to by ATel #: 3363, 3378, 3393, 5327, 5490, 15586

We report recent activities of the globular cluster M15. MAXI/GSC detected an X-ray burst at the scan transit centered at UT 2011-05-16T23:23:41. The burst was detected at least for 30.0 seconds within the 50.5 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) = (+322.27 deg, +12.14 deg) = (21 29 6, +12 08 23)(J2000)

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

(R.A., Dec) = (+322.31 deg, +11.95 deg) = (21 29 14, +11 57 11)(J2000)
(R.A., Dec) = (+322.06 deg, +12.10 deg) = (21 28 15, +12 06 9)(J2000)
(R.A., Dec) = (+322.24 deg, +12.32 deg) = (21 28 57, +12 19 23)(J2000)
(R.A., Dec) = (+322.48 deg, +12.17 deg) = (21 29 56, +12 10 24)(J2000)

There is additional systematic uncertainty of 0.2 degree (90% containment radius). This position is statistically consistent with that of the globular cluster M15, whose position is (R.A, Dec) = (322.49 deg, 12.17 deg). The flux was 370 +- 50 mCrab (1 sigma) in the 4 - 10 keV band.

On UT 2011-05-13T11:36:00, MAXI/GSC had also detected an signal with low significance at the position consistent with M15.

These two bursts suggest that at least one of X-ray sources in M15 has entered into an active state. M15 contains three known X-ray binaries; AC 211, M15 X-2, and M15 X-3. Archival MAXI data show flux increases at least twice at the end of April, 2011 (2011-04-26 and 2011-04-29), implying the beginning of this activity.

The daily light curve of M15 is available at the following URL:
http://maxi.riken.jp/top/index.php?cid=1&jname=J2129+121

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