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MAXI/GSC detection of a soft state transition in the faint X-ray transient Swift J1922.7-1716

ATel #3807; M. Morii (Tokyo Tech), H. Negoro (Nihon U.), S. Nakahira, M. Sugizaki, M. Serino, Y. Yamamoto, T. Sootome, T. Mihara, M. Matsuoka (RIKEN), S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), N. Kawai, K. Sugimori, R. Usui, T. Toizumi, Y. Aoki, S. Song (Tokyo Tech), A. Yoshida, K. Yamaoka (AGU), H. Tsunemi, M. Kimura, H. Kitayama (Osaka U.), M. Nakajima, F. Suwa, M. Asada, H. Sakakibara (Nihon U.), M. Shidatsu, Y. Ueda, K. Hiroi (Kyoto U.), Y. Tsuboi, T. Matsumura, K. Yamazaki (Chuo U.), M. Yamauchi, Y. Nishimura, T. Hanayama (Miyazaki U.) report on behalf of the MAXI team
on 15 Dec 2011; 16:11 UT
Credential Certification: Mikio Morii (morii@hp.phys.titech.ac.jp)

Subjects: X-ray, Binary, Neutron Star

Referred to by ATel #: 16451

The MAXI/GSC nova alert system triggered on a rapid increase in 2-4 keV X-ray flux from the neutron star low-mass X-ray binary Swift J1922.7-1716 during scan transits at 19:56 and 21:28 on December 14, 2011 (MJD=55909). The GSC data show that the transition has started since the late on December 13, 2011 (see MAXI GSC data: http://maxi.riken.jp/top/index.php?cid=000000000001&jname=J1922-172 ). The one-day averaged 2-4 keV flux increased from 6+-7 mCrab on December 13 to 42 +- 8 mCrab on December 14. Also note that the 15-50 keV flux obtained with Swift/BAT dropped rapidly on December 14 (Swift BAT data: http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/SWIFTJ1922.7-1716/ ). These data strongly suggest that the system has undergone a soft state transition. The current outburst has occurred since the beginning of August,2011 (ATel #3548, #3567). The source has showed an absorbed power-law spectrum (ATel #3567) except for duration of a possible type-I X-ray burst (ATel #3741, also see GCN #12522). Degenaar et al. propose that the source is classified as a faint X-ray transient (ATel #3741). We encourage followup observations of this source since it is interesting to see the nature of the soft state of this faint neutron star.

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