Low frequency quasi-periodic oscillations in the hard X-ray emission from Cygnus X-1 Original Research Article

The black-hole candidate Cyg X-1 was observed in a balloon flight experiment with large area Xenon-filled Multi-layer Proportional Counter (XMPC) telescope on April 5, 1992. The XMPC telescope sensitive in 20–100 keV range, observed Cyg X-1 in its hard state with an average counting rate of 51 counts per second for about 140 minutes. The timing analysis of the data show presence of Quasi-Periodic Oscillations (QPOs) in hard x-ray emission from the source. A broad QPO feature is seen clearly at a frequency of 0.06 Hz in the power density spectrum. Details of the results are presented and compared with reports of similar QPOs observed in Cyg X-1. The x-ray light curve also shows short time scale random intensity variations which are described by a shot noise model.