Abstract
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few-keV energy range by the Ariel V all-sky monitor (S. S. Holt, L. J. Kaluzienski, E. A. Boldt, and P. J. Serlemitsos, unpublished).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.
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References
Coe, M. J., Engel, A. R., and Quenby, J. J., Nature, 259, 544 (1976).
Hardly, D. M., Lightman, A. P., and Shapiro, S. L., Astrophys. J. Lett., 199, L153 (1975).
Mész´ros, P., Nature, 258, 531 (1915).
Holt, S. S., Kaluzienski, L. J., Boldt, E. A., and Serlemitsos, P. J., GSFC-preprint-X-661-76-37 (1976).
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SOMMER, M., MAURUS, H. & URBACH, R. The hard X-ray spectrum of Cyg X-1 during the transition in November 1975. Nature 263, 752–753 (1976). https://doi.org/10.1038/263752a0
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DOI: https://doi.org/10.1038/263752a0
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