Cygnus X-1 entered the soft state

ATel #3616; V. Grinberg (Remeis/ECAP/FAU), M. Boeck (Remeis/ECAP/FAU), K. Pottschmidt (CRESST/GSFC/UMBC), G. G. Pooley (MRAO), J. Wilms (Remeis/ECAP/FAU), M. A. Nowak (MIT-CXC), M. Cadolle Bel (ESA/ESAC), J. Rodriguez (CEA SAp/Lab. AIM Saclay), D. M. Marcu (CRESST/GSFC/UMBC), P. Uttley (Univ. Southampton), J. A. Tomsick (SSL/UC Berkeley), A. Bodaghee (SSL/UC Berkeley), S. B. Markoff (Univ. Amsterdam)
on 31 Aug 2011; 13:12 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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Radio and X-ray monitoring observations over the past few weeks - ATel #3534 (MAXI/GSC), ATel #3535 (AMI) and ATel #3546 (RATAN) - indicated Cyg X-1 entering the soft state. A plot of the behaviour of Cyg X-1 as observed with AMI, MAXI and BAT can be found under: http://www.mrao.cam.ac.uk/~guy/cx1/2011.ps. Our bi-weekly RXTE monitoring campaign of Cyg X-1 shows that the source transited into a soft state between August 13 and August 26. Three groups of observations were conducted: on July 29 (ObsID 96121-01-16-00/01/02/03/04, overall exposure 10 ks), August 13 (96121-01-17-00, 16 ks) and August 26/27 (96121-01-18-00/01/02/03, 6 ks). We model the 3--40 keV RXTE/PCA spectra with a broken power law, an iron line, an absorption component and, where required, a disk black body.

July 29:
The soft photon index of ~ 1.9-2.0 and the average 3.2-10 Hz time lag ranging between 3ms and 4ms point towards a softer mode of the hard state. The power spectra (PSDs) show a clear two-humped structure without a power law component and no black body component is required in the spectral fits.

August 13:
The spectra are similar to those of the July 29, with a slight softening of the photon index to 2.05-2.15 and an increase of the lag to 4-5 ms, while the PSDs remains two-humped with no or very small power law contribution and the black body is still not required. This behaviour is still indicative of a softer mode of the hard state or of the hard intermediate state.

August 26/27:
A multi-temperature black body component (diskbb) with a temperature of 0.35-0.50 keV is clearly required in the fits. The spectrum is steep, with a soft power law index between 2.7 and 2.9. The time lag is highly variable between 9 ms and -0.2 ms and the PSDs show a strong power law component as is typical for the soft state.

Taking into account the recent X-ray and radio monitoring data, we conclude from these three pointed observations that the source has transited into a soft state. This is especially interesting since the transition occurred so quickly after the end (ATel #3307) of the last prolonged soft state, which lasted from July 2010 to April 2011.