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RXTE Monitoring of Cyg X-1 in its current transitional state

ATEL # 2751; V. Grinberg (Dr. Karl Remeis Obs./ECAP), M. Boeck (Dr. Karl Remeis Obs./ECAP), K. Pottschmidt (CRESST/GSFC/UMBC), M.A. Nowak (MIT-CXC), P. Uttley (Southhampton), J.A. Tomsick (SSL/UC Berkeley), A. Bodaghee (SSL/UC Berkeley), S. Markoff (Amsterdam), G. Pooley (MRAO), I. Kreykenbohm (Dr. Karl Remeis Obs./ECAP), J. Wilms (Dr. Karl Remeis Obs./ECAP)
on 22 Jul 2010; 4:36 UT
Password Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

Subjects: Radio, X-ray, Binaries, Black Holes
Referred to by ATEL #: 2755

RXTE has performed pointed observations of Cyg X-1 on 2010-06-19/20, 2010-07-04, and 2010-07-19/20. Following the AGILE detections (ATel #2512, #2715) and subsequent observations indicating a softening of the source (Atel #2711, #2714, #2721, #2724, #2734) ending a years long hard state (Nowak et al., POS MQW7, 2008), we performed spectral and timing analysis of these observations.
The X-ray spectrum can be well described by the sum of an absorbed broken power law and a black body. Further state classification is performed based on the timing properties, which allow for a more accurate characterization than spectral parameters alone (Boeck et al., PoS MQW7, 2008; Pottschmidt et al., A&A, 357, L17, 2000). Power spectral densities and X-ray time lags were calculated from 4.5-5.8keV (soft band) and 9.5-15keV (hard band) PCA lightcurves with 2ms resolution, as well as a hardness intensity diagram (HID). The main parameters are summarized in the following table (radio flux densities are from the AMI large array):

observation begin obs [UT] exposure [ks] radio flux [mJy] soft photon index black body norm (from diskbb) [10^5] rms soft band [%] timelag (3.2-10Hz) [ms]
95121-01-13-00 2010-06-19 21:53:52 2.3 14.3 1.82 +/- 0.01 not required 28.8 3.0
95121-01-13-01 2010-06-19 23:31:28 2.1 14.4 1.83 +/- 0.01 not required 28.5 0.9
95121-01-13-02 2010-06-20 01:08:32 1.9 15.2 1.81 +/- 0.01 not required 27.4 1.7
95121-01-14-02 2010-07-04 00:27:28 2.1 9.0 2.65 +/- 0.03 1.3 +1.2/-0.5 20.6 8.0
95121-01-14-01 2010-07-04 02:05:20 2.5 5.3 2.68 +/-0.02 4.4 +1.5/-1.1 21.2 13
95121-01-14-00 2010-07-04 03:36:32 1.6 8.0 2.67 +/- 0.03 3.2 +1.5/-1.1 21.6 12
95121-01-15-00 2010-07-19 20:03:28 3.3 not available 3.01 +/- 0.02 13 +4/-3 32.7 0.9
95121-01-15-02 2010-07-19 23:12:32 1.5 2.8 2.88 +/- 0.03 3.4 +2.3/-1.5 31.2 1.7
95121-01-15-01 2010-07-20 02:20:32 1.2 not available 2.86 +/- 0.03 1.6 +1.7/-0.7 31.8 1.5

Following Wilms et al., A&A, 447, 245 (2006), on 2010-06-19/20 the spectral and timing properties are indicative of a hard state.
On 2010-07-04 the HID clearly reveals the source to be on its bright soft branch (Boeck et. al, PoS MQW7, 2008), as is confirmed by the soft photon index and the presence of disk emission. We note, however, that the PSD still shows Lorentzian components, the rms is low, and the X-ray time lags are enhanced, which are typical characteristics of a transitional state (Pottschmidt et al., A&A, 407, 1039, 2003). Note that during this phase the radio outflow and hard tails are usually the strongest (Wilms et al., ApJ, 663, L97, 2007; Cadolle Bel et al., A&A, 446, 591, 2006).
On 2010-07-19/20 the soft state is reached: the hardness of the source decreases compared to 2010-07-04 while the count rates are comparable. Both the soft photon index and the contribution of the disk emission increase further. A cut-off power law without further Lorentzian components offers a good description of the PSDs and the X-ray time lags are low.
Further monitoring of Cyg X-1 within the RXTE core program will continue.


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R. E. Rutledge , Editor
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