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Cygnus X-1 in the intermediate state

ATel #6021; V. Grinberg (MIT), M. Nowak (MIT), J. Wilms (Remeis/ECAP/FAU), G. G. Pooley (MRAO), K. Pottschmidt (CRESST/GSFC, UMBC), N. Hell (Remeis/ECAP/FAU, LLNL), J. A. Tomsick (SSL/UCB), P. Uttley (UVA), J. Rodriguez (CEA)
on 28 Mar 2014; 15:42 UT
Credential Certification: Victoria Grinberg (grinberg@space.mit.edu)

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

Referred to by ATel #: 6115, 6244, 6344, 7316

We follow up on ATel #5995 that announced Cygnus X-1 to be entering the hard state.

To assess the state of the source we analyze the publicly available MAXI and BAT data following the state definition method for Cyg X-1 introduced in Grinberg et al. 2013 (A&A 554, A88) that simultaneously uses MAXI count rates and hardnesses. For both instruments, we use satellite-orbit-wise resolved data (i.e., a measurement every 2-4 hours) as Cyg X-1 can show state transitions on time scales as short as hours (Boeck et al., 2011, A&A 533, A8) so that a daily average may be misleading for the true behavior of the source.

MAXI HIDs (see below for link to PDF) shows a trend toward the hard state, in agreement with the previous result of ATel #5995. However, for any given 3 day period within the last 18 days the individual MAXI observations cover all three states as defined on MAXI HIDs. The increase in the fraction of the soft state data points in the recent three days is not necessarily indicative of a return into the soft state, but does clearly demonstrate that the source behavior is currently highly variable.

BAT data can only be used to define the soft state, not to differentiate between intermediate and hard states. Cyg X-1 shows a clear trend away from the soft state (see below for link to PDF), with all measurement in the last 9 days above the soft state threshold of 0.09 cps/cm^2.

AMI radio data (at 15 GHz) showed a flare of ~22 mJy around MJD 56733. The radio flux has been at ~10 mJy since then, i.e., at a value typical outside of the soft state.

We conclude that the source has not fully transited into the hard state yet, but is rather predominantly in an intermediate state, punctuated by short excursions towards hard and soft states. No projections of the source's future behavior are currently possible, especially given the behavior of Cyg X-1 in the last ~4 years where a stable soft state was interrupted only by short hard states and failed soft to hard transitions. We encourage further monitoring and observations covering the intermediate state.

MAXI HIDs and BAT lightcurve plots used in this analysis are available at: