Abstract
Cygnus X-l, a strong X-ray source in a binary star system, is generally considered a good candidate for a black hole1. Its X-ray emission provides the best method of studying the physical processes near the collapsed object. The average energy spectrum (in the ‘low’ state) is remarkably stable2 and has been interpreted as the result of comptonization, that is Compton scattering of optical or UV photons in a very hot plasma3–5. We have measured the low-state spectrum on 26 October to 18 November 1977, over a wide energy range (3 keV–8 MeV). This is the first measurement to cover such a broad energy range and the first long-term average measurement above 300 keV. This spectrum agrees well with a single temperature comptonization model at low energy, but shows a significant excess at high energy (E>300 keV).
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Nolan, P., Gruber, D., Knight, F. et al. Hard X-ray spectrum of Cygnus X-l. Nature 293, 275–277 (1981). https://doi.org/10.1038/293275a0
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DOI: https://doi.org/10.1038/293275a0
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