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Cyg X-1 entered a transitional state, may be on its way from the soft state back to the hard state

ATel #3307; 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), P. Laurent (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), I. Kreykenbohm (Remeis/ECAP/FAU)
on 27 Apr 2011; 02:04 UT
Credential Certification: Katja Pottschmidt (katja@milkyway.gsfc.nasa.gov)

Subjects: Radio, X-ray, Gamma Ray, Binary, Black Hole

Referred to by ATel #: 3534, 3546, 3616, 3636, 7316

We report on recent RXTE, INTEGRAL, and AMI observations of the HMXB Cyg X-1. After having spent 2006 to mid-2010 in an extraordinarily hard state (Nowak et al., 2011, ApJ 728, 13) the source entered a soft state in summer 2010 (ATel #2711, #2714, #2715, #2721, #2724, #2734, #2751, #2755) which might now be coming to an end. The Swift-BAT hard X-ray flux started to recover, showing an increasing trend starting around 2011-04-18. Similarly the AMI radio flux recovered from being consistent with zero to almost its canonical hard state value of 15 mJy between 2011-04-09 and 2011-04-25. This picture is supported by our recent pointed X-ray observations:

The 20-500 keV INTEGRAL/ISGRI spectrum (66 ks) on 2011-04-11/12 can be described by a power law with a photon index of 2.63 +/- 0.02, consistent with results from our INTEGRAL Key Programme observations throughout the soft state. The 3-40 keV RXTE/PCA spectra (1.4-2.3 ks) can be described by a broken power law with a break at about 10 keV and photon indices PL1 and PL2 below and above the break, respectively, plus a black body component. We also determined rms variability values (for 0.008-256 Hz; narrower than the 0.002-256 Hz of ATel #2751 due to shorter exposure times) and timelags (for 3.2-10 Hz) from 4.5-5.8 keV and 9.5-15 keV PCA lightcurves. Summary of selected parameters (the disk parameters are not well constrained and therefore not included):

 
obs start [UT]       radio flux [mJy]    photon index PL1  soft band rms [%]  timelag [ms] 
2011-04-09                nd 
2011-04-12 04:12:32                       3.02 +/- 0.12         31.1          -0.3 +/- 0.9 
2011-04-12 05:50:24                       3.29 +/- 0.13         20.8           2.2 +/- 1.3 
2011-04-12 07:27:28                       2.95 +/- 0.15         26.0           4.5 +/- 1.4 
2011-04-12 09:05:36                       3.14 +/- 0.20         28.6           3.5 +/- 1.6 
2011-04-14 07:37:55       6.4 
2011-04-16 08:49:54       5.6 
2011-04-19 11:45:35       5.8 
2011-04-21 08:25:28       8.5 
2011-04-22 02:18:24                       2.71 +/- 0.08         20.2           6.8 +/- 1.7 
2011-04-22 03:55:28                       2.69 +/- 0.11         23.3           5.0 +/- 1.1 
2011-04-22 05:32:32                       2.76 +/- 0.13         22.8           4.6 +/- 1.7 
2011-04-22 05:49:53       6.4 
2011-04-22 07:08:32                       2.71 +/- 0.10         15.3           3.5 +/- 1.6 
2011-04-22 08:36:32                       2.69 +/- 0.12         20.0          11.6 +/- 2.1 
2011-04-24 02:19:38       6.5 
2011-04-25 08:57:06      12.1 
nd = no detection

The decreasing photon index between 2011-04-12 and 2011-04-22 (PL1 and PL2 are correlated; Wilms et al., 2006, A&A 447, 245), the comparatively low rms values, the radio flux turn-on/increase, and especially the enhanced timelag on 2011-04-22 are typical indicators that Cyg X-1 has entered a transitional state. The photon index PL1 on 2011-04-22 is consistent with the one measured for the RXTE monitoring observation on 2010-07-04, classified as transitional in ATel #2751. Abrupt parameter changes within the transitional state have been observed PL1~2.5 before (HIMS-to-SIMS-like transition, Boeck et al., 2011, A&A, in prep.): Cyg X-1 might soon reach this parameter regime.

Even if the source fails to reach the full hard state, this episode can be described as a reverse failed state transition, analogous to the previously observed failed state transitions from the hard to the soft state (Pottschmidt et al, 2003, A&A 407, 1039). Further monitoring of Cyg X-1 will be performed within the RXTE Core Program and within the INTEGRAL Key Programme, as well as with the AMI Large Array. We encourage additional multi-wavelength observations in order to reveal a more detailed view of a soft to hard state transition for a persistent source.