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MAXI/GSC detection of an X-ray outburst from NGC6440

ATEL # 2360; M. Suzuki (JAXA), H. Negoro (Nihon U), S. Nakahira (AGU), T. Mihara, M. Kohama, Y.E. Nakagawa, M. Sugizaki, T. Yamamoto (RIKEN), M. Matsuoka, K. Kawasaki, S. Ueno, H. Tomida, M. Ishikawa (JAXA), N. Kawai, M. Morii, K. Sugimori (Tokyo Tech), A. Yoshida, K.Yamaoka (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.) report on behalf of the MAXI team
on 28 Dec 2009; 12:13 UT
Password Certification: Motoko Suzuki (

Subjects: X-ray, Binaries, Globular Clusters, Transients

MAXI/GSC detected an X-ray outburst from the globular cluster, NGC6440. Observed flux of the source in the 4-10 keV band was about 250 mCrab on 2009 December 22.8 UT. MAXI/GSC could not detect the source before 2009 December 6 UT, and the X-ray flux level in the same energy band from the same region was lower than about 10 mCrab. We could not observe the beginning of the outburst due to the avoidance of solar X-ray. In addition, MAXI/GSC detected a rapid flare, that looks like a type-I X-ray burst, from the source during the transit starting at 05:48:20 (UT) on December 28. NGC6440 is a globular cluster, which is known as containing at least two transient X-ray sources. A bright transient was observed a few times (for example In't Zand et al. 1999, A&A 345, 100; Verbunt et al. 2000, A&A 359, 960; Markwardt and Swank 2005, Atel #495). Recently a new transient was found (Heinke et al. 2009, Atel #2139; Altamirano et al. 2009, arXiv:0911.0435v1) in NGC6440, but observed flux is rather low (7 mCrab, Heinke et al. 2009, Atel #2143). MAXI is unable to resolve separately these sources due to its spatial resolution of about 1 deg. The X-ray flux of this outburst, however, suggests the source is the former transient known as SAX J1748.9-2021. You may see MAXI light curves through the below URL: (Note that X-ray counts, especially in the soft X-ray light curve, are much affected by the solar X-ray contamination from December 22 to 23.) MAXI will continue to monitor this source. Follow-up observations with multi-wavelength are encouraged.

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R. E. Rutledge , Editor
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