Re-brightening of MAXI J0556-332, a transient NS X-ray binary, detected by MAXI/GSC

ATel #4524; M. Sugizaki, M. Matsuoka (RIKEN), H. Negoro (Nihon U.), S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa, K. Yamaoka (JAXA), T. Mihara, M. Serino, K. Morihana, T. Yamamoto, J. Sugimoto (RIKEN), N. Kawai, M. Morii, R. Usui, K. Ishikawa (Tokyo Tech), A. Yoshida, T. Sakamoto (AGU), H. Tsunemi, M. Kimura (Osaka U.), M. Nakajima, M. Asada (Nihon U.), Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.), Y. Tsuboi, M. Higa (Chuo U.) M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.) on behalf of the MAXI team
on 28 Oct 2012; 13:21 UT
Credential Certification: Mutsumi Sugizaki (sugizaki@riken.jp)

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MAXI/GSC now detects an X-ray brightening of a transient NS X-ray binary, MAXI J0556-332 in 2012 October 27.

The source was discovered on 2011 January 11 and suggested to be a transient neutron-star binary from multiwavelength observation results (ATEL #3102, #3103, #3104, #3106, #3110, #3112, #3116, #3119, #3327, #3328, #3349, #3650). The X-ray activity continued with a flux over 30 mCrab (6x10^-10 erg cm^-2 s^-1 in 2-10 keV band) for about 1.5 year until 2012 May. It then turned below the MAXI/GSC detection limit.

The Swift/XRT continuous monitoring revealed that the flux suddenly dropped into ~1x10^-12 erg cm^-2 s^-1 by two orders of magnitude for about a week from 2012 April 21 (MJD=56038), which suggests the source changed to the quiescent phase. If this object is a NS-LMXB, this suggests that the accretion rate onto the neutron star decreased through the point of the Alfven radius over the co-rotation radius (see Matsuoka and Asai 2012, http://arxiv.org/abs/1210.2586 ).

The latest X-ray light curve obtained by MAXI/GSC survey shows that the X-ray flux gradually increased since 2012 October 22 (MJD=56222) and reached about 12 mCrab (2x10^-10 erg cm^-2 s^-1 in 2-10 keV band) on October 27 (MJD=56227). This implies that the source would be now in the active phase again as seen in 2011.

Further observations with multiwavelength are encouraged.