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RXTE-ASM detects the start of a possible state transition in Cygnus X-1

ATEL # 2714; A. Rushton (ESO), V. Dhawan (NRAO), R. Fender (Uni. Southampton), M. Garrett (ASTRON), T. Maccarone (Uni. Southampton), James Miller-Jones (NRAO), Z. Paragi (JIVE), G. Pooley (Uni. Cambridge), R. Spencer (Jodrell Bank) & V. Tudose (ASTRON)
on 1 Jul 2010; 17:46 UT
Password Certification: Anthony Rushton (Anthony.Rushton@Manchester.ac.uk)

Subjects: Radio, X-ray, Request for Observations, Binaries, Black Holes, Transients
Referred to by ATEL #: 2715, 2721, 2724, 2734, 2751

Recent data from the All Sky Monitor (ASM) onboard the RXTE satellite has detected an increase in X-rays within the 2-12 keV band. The ASM measured an increase in X-rays starting around 23 June 2010 (MJD 55370) and peaking on 1 July 2010 (MJD 55377.8) with a brightness of 51.92 c/s (688 mCrab) and HR2 = 1.088. In-addition, monitoring with the SWIFT-BAT (15-150 keV) has shown no significant increase, but radio observations with the AMI radio interferometer (15 GHz), have not yet shown any significant radio flaring either.

These results are therefore consistent with recent measurements taken with MAXI/GSC (Atel #2711), suggesting the start of a soft X-ray flare. Interestingly, it is noted that this X-ray flare isn't a smooth rise and a short dip was also observed on 29 June 2010 (MJD 55375.9).

We conclude that this is indicative of a hard-to-soft state transition due to the significant softening of the X-ray spectrum; however, failed state transitions have been known to occur in Cygnus X-1, such as the short flare occurring in June 2009 which reached a peak X-ray brightness of ~48 c/s (636 mCrab), in the 2-12 keV band, before falling back into the hard-state.

Table 1: RXTE-ASM observations (2-12 keV) of Cygnus X-1 in June/July 2010.

MJD ASM c/s error mCrab error HR2
55377.8 51.92 1.13 687.72 15.01 1.09
55376.84 38.47 0.88 509.56 11.6 1.15
55375.93 29.24 0.8 387.25 10.64 1.33
55374.74 37.82 0.6 500.93 7.89 1.1
55374.19 34.66 0.82 459.01 10.92 1.15
55373.11 25.52 0.58 338.06 7.63 1.29
55371.8 29.03 0.76 384.55 10.12 1.22

Results provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC


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R. E. Rutledge , Editor
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