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Gamma-Ray Spectra and Variability of Cygnus Z-1 Observed by BATSEWe present new BATSE earth occultation observations of the 25 keV-1.8 MeV spectrum and variability of Cygnus X-1 made between August 1993 and May 1994. We observed that the normal soft gamma ray spectrum (gamma2) of Cygnus X-1 has two components: a Comptonized part seen below 300 keV, and a high-energy tail in the 0.3 - 2 MeV range. We interpret it in terms of a two-layer region, consisting of a high-energy core (with an equivalent electron temperature of approximately 210-250 keV) near the event horizon, embedded in an about 50 keV corona. In this scenario, the observed 25-300 keV photons were produced by Compton scattering of soft photons (about 0.5 keV) by the hot electrons in the outer corona. These same hard x rays were further up-scattered by a population of energetic electrons in the inner core, producing the spectral tail above 300 keV. Cygnus X-1 went through an extended sequence of transitions between August 1993 and May 1994, when the 45-140 keV flux first decreased steadily from approximately gamma2 to roughly one-quarter of its intensity over a period of about 140 days. The flux remained at this low level for about 40 days before returning, swiftly (approximately 20 days) to approximately the initial gamma2 level. During the transition, the spectrum evolved to a shape consistent with either a power law with photon index of about 2.6 or a single temperature Compton model with electron temperature kT = 110 +/- 11 keV, and optical depth t = 0.40 +/- 0.06, and then returned essentially to the original gamma2 spectrum at the end of the active period. The overall cooling of the system during the low flux period may be due to an increase in the soft photon population which effectively quenched the hot electrons in these regions through Compton scattering.
Document ID
19990068042
Acquisition Source
Marshall Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Ling, J. C.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA United States)
Wheaton, William A.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA United States)
Wallyn, P.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA United States)
Mahoney, W. .
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA United States)
Paciesas, W. S.
(Alabama Univ. Huntsville, AL United States)
Harmon, B. A.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Fishman, G. J.
(NASA Marshall Space Flight Center Huntsville, AL United States)
Zhang, S. N.
(Universities Space Research Association Huntsville, AL United States)
Hua, X. M.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
August 19, 2013
Publication Date
January 1, 1998
Subject Category
Space Radiation
Distribution Limits
Public
Copyright
Other

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