MAXI/GSC detected a rapid soft X-ray brightening in Cyg X-1

ATel #2711; H. Negoro (Nihon U.), N. Kawai (Tokyo Tech), Y. Ueda (Kyoto U.) K. Kawasaki, S. Ueno, H. Tomida, M. Kohama, M. Ishikawa (JAXA), T. Mihara, Y. E. Nakagawa, M. Sugizaki, M. Suzuki, T. Yamamoto, T. Sootome, M. Matsuoka (RIKEN), M. Morii, K. Sugimori (Tokyo Tech), A. Yoshida, K. Yamaoka, S. Nakahira (AGU), H. Tsunemi, M. Kimura (Osaka U.), M. Nakajima H. Ozawa, F. Suwa (Nihon U.), N. Isobe, S. Eguchi, K. Hiroi (Kyoto U.), A. Daikyuji (Miyazaki U.), A. Uzawa, T. Matsumura, K. Yamazaki (Chuo U.) report on behalf of the MAXI team
on 1 Jul 2010; 11:35 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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Recent MAXI/GSC data show a gradual spectral softening of the black hole candidate Cyg X-1 since around 2010 June 10. The soft X-ray flux (2-4 keV) once increased rapidly to ~600 mCrab on 2010 June 27 and 28, but it went back to a more common flux level of the low/hard state ~300 mCrab on June 29. The latest data on 2010 July 1, however, show a rapid increase in soft X-ray flux again, reaching ~1.2 Crab (2-4 keV). A preliminary spectral analysis shows that the spectrum on July 1 is well fitted with an absorbed power-law with a photon index of about 2.3, strongly suggesting that the source is undergoing a hard-to-soft transition. Observations in other wavebands, in particular gamma-ray and radio observations, are encouraged.

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