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

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 ~ 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 to increase by ~ 30%. Also conducted was timing analysis over a frequency range of 0.001–10 Hz. As the source became brighter in soft X-rays, the characteristic frequency of hard X-ray variation increased from 0.03 to 0.3 Hz, while the fractional hard X-ray variation integrated over 0.001–0.01 Hz decreased by a factor of ~ 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.