Long term variability of Cygnus X-1

V. State definitions with all sky monitors

¹ Dr. Karl−Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich Alexander Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: victoria.grinberg@fau.de
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

Abstract

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.