Frequency-resolved spectroscopy of Cyg X-1: fast variability of the reflected emission in the soft state

Abstract

Using RXTE/PCA data, we study the fast variability of the reflected emission in the soft spectral state of Cyg X‐1 by means of Fourier frequency‐resolved spectroscopy. We find that the rms amplitude of variations of the reflected emission has the same frequency dependence as the primary radiation down to time‐scales of ≲30–50 ms. This might indicate that the reflected flux reproduces, with nearly flat response, variations of the primary emission. Such behaviour differs notably from that of the hard spectral state, in which variations of the reflected flux are significantly suppressed in comparison with the primary emission, on time‐scales shorter than ∼0.5–1 s. If related to the finite light‐crossing time of the reflector, these results suggest that the characteristic size of the reflector, presumably an optically thick accretion disc, in the hard spectral state is larger by a factor of ≳5–10 than in the soft spectral state. Modelling the transfer function of the disc, we estimate the inner radius of the accretion disc to be Rin∼100Rg in the hard state and Rin≲10Rg in the soft state for a 10‐M⊙ black hole.

Excitation of low-frequency QPOs in black hole accretion

“…For frequencies characteristic of the oscillations (around 1 Hz), the ISAF must extend to R out ∼ 100R s . This is comparable to the extent of the ISAF in the low/hard state in Cygnus X-1, as deduced by Gilfanov et al (2000) from Fourier frequency-resolved spectroscopy.…”

“…The dependence of frequency on accretion rate would then imply an increase of the transition radius with decreasing accretion rate. Supporting evidence for such a physical relation is found in the correlation between reflection indicators (Fe line strength and amplitude of the Compton reflection hump) and the strength of the soft component in the X-ray spectrum with QPO frequency (Zdziarski et al 1999;Gilfanov et al 2000).…”

X-Ray Emission from Black-Hole Binaries

“…As suggested in [45,19], the variability of the reflected emission can be studied using the methods of frequency-resolved spectroscopy (Sect. 6.4).…”

“…1. Energy spectra of a black hole Cyg X-1 (left) and a neutron star 4U 1705-44 (right) in the soft and hard spectral states (left panel adopted from [19]). …”

The transition from a cool disk to an ion supported flow

“…Signatures of a disk transition radius have recently been found in the power density spectra of Cyg X-1 (Churazov et al 2001;Gilfanov et al 2000). From an analysis of the reflection component Di Salvo et al (2001) showed that in Cyg X-1 the transition radius R tr is located between 10 R S and 70 R S , if the observed reflected spectrum is due to a smeared component, or 6 R S < R tr < 20 R S if the reflection is unsmeared, e.g.…”