The variable X-ray spectrum of Cyg XR-1

Abstract

The variability of the X-ray spectrum of the discrete source Cyg XR-1 (c~ = 19 h 56m) = q-35~ is reviewed. The variations observed in the energy region accessible to balloon borne detectors (energies greater than 20 keV) can be explained by assuming them to be caused by the eclipsing properties of a binary system. It is suggested that the system is composed of a source of small angular extent having a spectrum similar to that of a black body at approximately 1.5 x 108 K (kT= 12.5 keV) and a non X-radiating companion which eclipses it at intervals of 2.9850 days. The system would be surrounded by an X-radiating plasma whose photon flux between 1 and 100 keV can be approximated by a power law spectrum whose exponent is --1.7.The discrete X-ray source located at e= 19h56 m, 6= +35.~ and ordinarily designated as Cyg XR-1 (Friedman et al., 1967) is one of the brightest celestial objects in the X-ray region above 20 keV. Because of this, observations have been made of the object since 1964 by balloon borne detectors as well as by rocket borne instrumentation. Although the first observations of the object in the X-ray region below 10 keV led Byram et al. (1966) to make the suggestion that the source was of variable intensity, a more frequent series of measurements taken by balloon borne equipment gave the first convincing proof of the intrinsic variability of the object (see Overbeck and Tananbaum, 1968a), at least in the region above 20 keV. It is profitable to review the variability of the object in the energy regions specified above from the standpoint of deciding whether any regularity, either in spectral shape vs. intensity or in intensity vs. time, can be deduced from the observations.

48. High Energy Astrophysics

“…The radio source is coincident with the 9th mag BOI, HDE 226868 (Webster and Murdin (25), Bolton (26)) which was found to be a spectroscopic binary with a 5-6 day period. The X-ray emission shows no evidence of the 5-6 day binary motion (Tananbaum et al (27)) in spite of earlier claims that it did (Dolan (28,29)). Later spectroscopic studies revealed at least one other spectral line, He n-4686, out of phase with the remainder (Brucato and Kristian (30)).…”