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Title: Cygnus X-1 - a (failed?) state transition

ATel #6244; 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. Tomsick (SSL/UCB), P. Uttley (UvA), J. Rodriguez (CEA)
on 17 Jun 2014; 16:01 UT
Credential Certification: Victoria Grinberg (grinberg@space.mit.edu)

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

Referred to by ATel #: 6344

We have used the publicly available MAXI and BAT data of Cyg X-1 to follow the recent soft to hard transition (ATels #5995, #6021) and the following hard state (ATels #6115, #6119). In particular, we are using the state definition method for Cyg X-1 introduced in Grinberg et al. 2013 (A&A 554, A88) using satellite-orbit-wise resolved data from both monitoring instruments to analyze the behavior of the source on timescales of a few hours.

After a week-long rather stable hard state, the source currently shows characteristics typical of the intermediate state. In particular this is seen in the MAXI hardness-intensity-diagram (see figure below) and supported by the enhanced radio emission (see figure below). The BAT hard X-ray lightcurve shows a slight decrease, but has not reached the soft state levels (see figure below).

The current behavior is not yet indicative of a soft state and we cannot predict whether the source will transit fully or return into the hard state. We therefore currently advise against observations that strongly rely on a particular source state and encourage monitoring of the possible (failed) state transition.

MAXI HIDs and BAT lightcurve plots used in this analysis are available at:
http://www.sternwarte.uni-erlangen.de/~grinberg/perm/201406_atel_maxi_hid.pdf
http://www.sternwarte.uni-erlangen.de/~grinberg/perm/201406_atel_bat_lc.pdf

Current radio monitoring data are available at:
http://www.mrao.cam.ac.uk/~guy/cx1/2014.ps