The hard X-ray spectrum of Cyg X-1 during the transition in November 1975

WE wish to report an observation of the hard X-ray spectrum of Cyg X-1 during a transition to the high state in November 1975. The measurement was obtained with a balloon-borne X-ray detector in the 25-150 keV range. This third upward transition of Cyg X-1 in 1975 was also detected in the few-keV energy range by the Ariel V all-sky monitor (S. S. Holt, L. J. Kaluzienski, E. A. Boldt, and P. J. Serlemitsos, unpublished). Our high energy data seem to confirm the characteristic spectral time variation of Cyg X-1: a soft X-ray intensity increase is correlated with a decrease of the hard X-ray intensity. The similarity of Cyg X-1 and the transient source A0620-00 with respect to such spectral variation has been pointed out by Coe et al.¹.