Volume 554, June 2013
|Number of page(s)||12|
|Published online||07 June 2013|
Long term variability of Cygnus X-1
V. State definitions with all sky monitors
Dr. Karl−Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics,
Friedrich Alexander Universität Erlangen-Nürnberg,
2 Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA
3 CRESST, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
4 NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
5 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
6 MIT−CXC, NE80−6077, 77 Mass. Ave., Cambridge, MA 02139, USA
7 Laboratoire AIM, UMR 7158, CEA/DSM − CNRS − Université Paris Diderot, IRFU/SAp, 91191 Gif-sur-Yvette, France
8 Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA
9 European Space Astronomy Centre (ESA/ESAC), Science Operations Department, PO Box 78, 28691 Villañueva de la Canãda, Madrid, Spain
10 Department of Physics, La Sierra University, Riverside, CA 92515, USA
11 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
12 Astrophysics, Cavendish Laboratory, University of Cambridge, CB3 0HE, UK
13 Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, 9500 Gilman Drive, CA 92093-0424, USA
14 ZP 12, NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
Received: 18 January 2013
Accepted: 5 March 2013
We present a scheme for determining the spectral state of the canonical black hole Cyg X-1 using data from previous and current X-ray all sky monitors (RXTE-ASM, Swift-BAT, MAXI, and Fermi-GBM). Determinations of the hard/intermediate and soft state agree to better than 10% between different monitors, facilitating the determination of the state and its context for any observation of the source, potentially over the lifetimes of different individual monitors. A separation of the hard and the intermediate states, which strongly differ in their spectral shape and short-term timing behavior, is only possible when data in the soft X-rays (<5 keV) are available. A statistical analysis of the states confirms the different activity patterns of the source (e.g., month- to year-long hard-state periods or phases during which numerous transitions occur). It also shows that the hard and soft states are stable, with the probability of Cyg X-1 remaining in a given state for at least one week to be larger than 85% in the hard state and larger than 75% in the soft state. Intermediate states are short lived, with a 50% probability that the source leaves the intermediate state within three days. Reliable detection of these potentially short-lived events is only possible with monitor data that have a time resolution better than 1 d.
Key words: X-rays: binaries / stars: individual: Cygnus X-1 / binaries: close
© ESO, 2013
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