The Estimated Distance to Cygnus X-1 Based on its Low-Energy X-Ray Spectrum

Cygnus X-1 was observed in the X-ray energy interval 0.2-1.2 keV with a focusingcollector payload flown aboard a sounding rocket. Virtually no flux is seen below 0.5 keV, which is consistent with absorbing matter equivalent to 1.6 X 1021 H atoms crn along the line of sight. The 21-cm radiation proffles in the Cygnus region indicate that this amount of hydrogen represents a distance of about 1 kpc to the source. At this distance, the intrinsic luminosity of Cyg X-1 is 3 X 1036 ergs 1 between 0.5 and 100 keV.