Energy spectrum and time variations of hard X-rays from Cyg X-1

Abstract

Experimental results on the intensity, energy spectrum and time variations in hard X-ray emission from Cyg X-1 based on a balloon observation made on 1971, April 6 from Hyderabad (India) are described. The average energy spectrum of Cyg X-1 in the 22-154 keV interval on 1971 April 6 is best represented by a power law dN/dE=(5.41±1.53)E ^{-(1.92±0.10)} photons cm^-2s^-1 keV^-1 which is in very good agreement with the spectrum of Cyg X-1 derived from an earlier observation made by us on 1969 April 16 in the 25-151 keV band and given by dN/dE=(3.54±2.44)E ^{-(1.89±0.22)} photons cm^-2s^-1 keV^-1. A thermal bremsstrahlung spectrum fails to give a good fit over the entire energy range for both the observations. Comparison with the observations of other investigators shows that almost all balloon experiments consistently give a spectrum of ∼ E ^-2, while below 20 keV the spectrum varies from E ^-1.7 to E ^-5. There is some indication of a break in the Cyg X-1 spectrum around 20 keV. Spectral analysis of data in different time intervals for the 1971 April 6 flight demonstrates that while the source intensity varies over time scales of a few minutes, there is no appreciable variation in the spectral slope. Analysis of various hard X-ray observations for long term variations shows that over a period of about a week the intensity of Cyg X-1 varies upto a factor of four. The binary model proposed by Dolan is examined and the difficulties in explaining the observed features of Cyg X-1 by this model are pointed out.