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