Abstract
THE many X-ray observations of the black hole candidate Cyg X-1 show a variable power-law spectrum (see refs 1–8) 
where d is the distance to the source. During the ‘low state’, in which the source spends ∼ 90% of its time7, p ≃ 0.5 and the power-law spectrum extends over 1 keV ≲ E ≲ 100 keV, with a knee suggested at ∼ 100 keV. During the transient ‘high state’, which persists for weeks to months, the total luminosity at ≳ 1 keV increases by a factor 5–10 and the spectrum steepens to 2 ≲ p ≲ 4 over the range 1 keV ≲ E ≲ 10 keV. A hard excess above this steep slope is visible at > 10 keV, which contains ∼10% of the total energy. In this paper we glean some information about the physics of the source by interpreting this spectrum as being caused by inverse Compton scattering by nonrelativistic electrons at a temperature Te ≃ 109K.
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EARDLEY, D., LIGHTMAN, A. Inverse Compton spectra and the spectrum of Cygnus X-1. Nature 262, 196–197 (1976). https://doi.org/10.1038/262196a0
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DOI: https://doi.org/10.1038/262196a0
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