Unusual soft X-ray activity of Cygnus X-1 detected with MAXI/GSC

ATel #10322; H. Negoro, A. Sakamaki, T. Kawase (Nihon U.), K. Makishima (RIKEN), S. Ueno, H. Tomida, N. Isobe, M. Ishikawa, Y. Sugawara (JAXA), T. Mihara, M. Sugizaki, M. Serino, S. Nakahira, W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, K. Morita, Y. Muraki (Tokyo Tech), A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka (AGU), H. Tsunemi, T. Yoneyama (Osaka U.), M. Nakajima (Nihon U.), Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), M. Yamauchi, C. Hanyu, K. Hidaka (Miyazaki U.), K. Yamaoka (Nagoya U.), T. Kawamuro (NAOJ)
on 26 Apr 2017; 03:23 UT
Credential Certification: Hitoshi Negoro (negoro@phys.cst.nihon-u.ac.jp)

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We report recent, unusual soft X-ray activity of Cygnus X-1. MAXI/GSC data show that the 2-10 keV X-ray flux from the source has gradually increased since the beginning of March 2017. In the period from 2016 April 25 (MJD 57503) to 2016 October 31 (MJD 57692) when the source was in the hard state, the 2-4 keV and 4-10 keV X-ray fluxes were 0.345 +/- 0.002 ph/s/cm2 (~210 mCrab) and 0.365 +/- 0.001 ph/s/cm2 (~310 mCrab), respectively. In a recent period from March 18-25 (MJD 57830 to 57837), these fluxes increased to 0.848 +/- 0.011 ph/s/cm2 (~510 mCrab), and 0.548 +/- 0.008 ph/s/cm2 (~480 mCrab), respectively. On the other hand, the 10-20 keV X-ray flux did not change significantly from 0.171 +/- 0.001 ph/s/cm2 (~480 mCrab) to 0.173 +/- 0.006 ph/s/cm2 between those periods.

After the period from March 26 (MJD 57838) to April 20 (MJD 57863), when the source was observed only with degraded cameras and no calibrated data were provided yet, the source further showed rapid time variations in the soft X-ray bands on a timescale of hours. The highest soft X-ray fluxes so far observed in one day are 1.248 +/- 0.044 ph/s/cm2 (~750 mCrab), 0.758 +/- 0.027 ph/s/cm2 (~660 mCrab), and 0.248 +/- 0.020 ph/s/cm2 (~690 mCrab) in the 2-4 keV, 4-10 keV, and 10-20 keV band, respectively, on April 25 (MJD 57868), and those in one scan transit typically for 45 sec are 2.20 +/- 0.19 ph/s/cm2 (~1.3 Crab), 0.94 +/- 0.10 ph/s/cm2 (~820 mCrab), and 0.21 +/- 0.06 ph/s/cm2 (~580 mCrab), respectively, in the scan transit at 05:08 UT on April 24.

A similar soft X-ray increase was observed at the beginning of the soft state in 2010 (Negoro et al. ATel. #2711). Thus, this soft X-ray activity may indicate the beginning of the soft state. At present, however, hard X-ray decreases usually accompanied with the hard-to-soft state transition have not yet been observed either in the MAXI/GSC data, or in Swift/BAT data (https://swift.gsfc.nasa.gov/results/transients/CygX-1/). On April 25, those hard X-ray fluxes were likely to increase.

Finally we note that the soft state in Cyg X-1 is exceptional in comparison with that in other black hole candidates. A highly variable soft component was usually observed (e.g., Sugimoto et al. 2016, PASJ, 68, S17), and very bright flares were also detected (Gierlinski and Zdziarski 2003, MNRAS, 343, L84), which were difficult to understand in the frame work of the standard optically thick and geometrically thin accretion disk model as the origin of the soft component. A typical soft state observed in other black hole candidates are rarely observed (e.g., Kawano et al. 2017, PASJ, 69, 36). Uncorrelated variations in the soft and hard energy bands observed this time also raise a question about the origin of hard X-rays produced by Compton up-scattering.
Followup and monitoring multi-wavelength observations are encouraged.

MAXI/GSC Public Data Product of Cyg X-1