Spectral and Timing Studies of Cyg X-1 in the Low/Hard State with Suzaku

Abstract

From 2005 to 2009, 25 observations of Cyg X-1 were performed with Suzaku, achieving a total exposure of 446 ks. In all observations, the source was found in the low/hard state, while the 1.5-12.0 keV count rate of the All-Sky Monitor onboard RXTE varied by a factor of $\sim\ $ 3. In each observation, the 10-60 keV HXD-PIN spectrum and the 60-400 keV HXD-GSO spectrum were fitted successfully by a thermal Comptonization model plus reflection by a thick neutral material. As the soft X-ray intensity increased, the Compton $y$ -parameter was found to decrease from 1.0 to 0.6, while the solid angle of reflection increased by $\sim\ $ 30%. Also conducted was a timing analysis over a frequency range of 10 $^{-3}$ -10 Hz. As the source became brighter in soft X-rays, the characteristic frequency of the hard X-ray variation increased from 0.03 to 0.3 Hz, while the fractional hard X-ray variation integrated over 10 $^{-3}$ -10 $^{-2}$ Hz decreased by a factor of $\sim\ $ 5. The signals in the 60-200 keV band were generally found to vary on shorter time scales than those in the 10-60 keV band. These spectral and timing results can be consistently interpreted by presuming that increases in the mass accretion rate cause the Comptonizing hot corona to shrink, while the optically-thick disk to intrude deeper therein.