Cyg X-1 transiting into the hard state
ATel #7316; V. Grinberg (MIT), G. G. Pooley (MRAO), M. Cadolle Bel (LMU), P. Laurent (APC/CEA), M. A. Nowak (MIT), K. Pottschmidt (CRESST/GSFC, UMBC), J. Rodriguez (AIM/CEA), J. Wilms (Remeis/ECAP/FAU)
on 30 Mar 2015; 18:47 UT
Credential Certification: Victoria Grinberg (grinberg@space.mit.edu)
Subjects: Radio, X-ray, Binary, Black Hole
We detect a steep increase in AMI-LA 15 GHz radio flux from the high mass X-ray binary Cyg X-1 in the last ~20 days. The source has spent most of the previous years in the (low radio flux) soft state (ATel #2711 for the start of the period) interrupted by only short flaring and hard state episodes (e.g., ATels #3307,#3534, #5995, #6021). The recent increase in radio flux follows a previous flaring episode (~ MJD 57000-57050) that remained in the intermediate state and did not reach a hard state according to the X-ray all sky monitor based state definitions of Grinberg et al. 2013 (A&A 554, A88) and a month-long soft state with very low radio flux (see Fig. http://www.sternwarte.uni-erlangen.de/~grinberg/perm/201503_cygx1_atel_radio.pdf ).
To further assess the behavior of the source, we analyzed satellite orbit-wise MAXI and BAT data (see Figs. http://www.sternwarte.uni-erlangen.de/~grinberg/perm/201503_cygx1_atel_bat.pdf , http://www.sternwarte.uni-erlangen.de/~grinberg/perm/201503_cygx1_atel_maxi.pdf ). The source shows a state transition from soft to hard state in the last ~6 days. We note especially that the observations from the last ~3 days clearly populate the hard state region of the MAXI hardness-intensity-diagram.
We encourage multiwavelength monitoring of the transition.
Judging from previous erratic behavior of the source it is not yet clear whether the transition will result in a persistent hard state. We therefore do not yet encourage observations that require a persistent hard state.
