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          Institute: MPI für extraterrestrische Physik     Collection: High Energy Astrophysics at MPE     Display Documents

ID: 5855.0, MPI für extraterrestrische Physik / High Energy Astrophysics at MPE
The soft gamma-ray spectral variability of Cygnus X-1
Authors:McConnell, M. L.; Zdziarski, A. A.; Bennett, K.; Bloemen, H.; Collmar, W.; Hermsen, W.; Kuiper, L.; Paciesas, W.; Phlips, B. F.; Poutanen, J.; Ryan, J. M.; Schönfelder, V.; Steinle, H.; Strong, A. W.
Date of Publication (YYYY-MM-DD):2002-06-20
Title of Journal:The Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:2
Start Page:984
End Page:995
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV gamma-ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder gamma-ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft gamma-ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of similar to4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.
Free Keywords:accretion; accretion disks; black hole physics; gamma rays : observations; stars : individual (Cygnus X-1); X-rays : stars
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für extraterrestrische Physik
External Affiliations:Univ New Hampshire, Ctr Space Sci, Durham, NH 03824 USA; Univ New Hampshire, Ctr Space Sci, Durham, NH 03824 USA; Nicolaus Copernicus Astron Ctr, PL-00716 Warsaw, Poland; European Space Agcy, ESTEC, Div Astrophys, NL-2200 AG Noordwijk, Netherlands; SRON, NL-3584 CA Utrecht, Netherlands; Max Planck Inst Extraterr Phys, D-85740 Garching, Germany; Univ Alabama, Huntsville, AL 35899 USA; USN, Res Lab, Washington, DC 20375 USA; Univ Oulu, Astron Div, Oulu 90014, Finland
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