Gursky, H.; Gorenstein, P.; Kerr, F. J.; Grayzeck, E. J. (1971). "The Estimated Distance to Cygnus X-1 Based on its Low-Energy X-Ray Spectrum". Astrophysical Journal. 167: L15. Bibcode:1971ApJ...167L..15G. doi:10.1086/180751.
Gursky, H.; Gorenstein, P.; Kerr, F. J.; Grayzeck, E. J. (1971). "The Estimated Distance to Cygnus X-1 Based on its Low-Energy X-Ray Spectrum". Astrophysical Journal. 167: L15. Bibcode:1971ApJ...167L..15G. doi:10.1086/180751.