# Monitoring the short-term variability of Cyg X-1 with RXTE

### K. Pottschmidt, J. Wilms, R. Staubert

### M.A. Nowak

### J.B. Dove

### W.A. Heindl

### D.M. Smith

## Abstract

For stellar black hole candidates several distinct states can be identified
that differ in their general spectral and temporal properties. Based mainly
on spectral arguments the states have been associated with different
accretion rates and geometries. With broad band instruments like RXTE it
is now possible to study the canonical black hole states with high time
resolution over a time base of several years.
We analyzed the hard, i.e., most common, state of Cyg X-1,
using RXTE observations that were performed from 1996 to 1999. The
focus of our work lies on parameters and functions characterizing the
short-term variability ( < 1000 sec), like:

a) Power Spectral Densities (PSDs).

b) Linear State Space Models. According to these stochastic models, one
typical relaxation timescale can be attributed to the hard state
lightcurves of Cyg X-1.

c) Fourier frequency dependent quantities describing the relationship
between different energy bands, namely coherence functions and timelags.

We find that the timing properties show a considerable variability, even
though the state does not change. This has already been noted for PSDs, but
is probably even more pronounced for the timelags. We also show how the
changes relate to spectral parameters such as power law indices, coronal
temperatures or reflection hump strengths.

File translated from T_{E}X by T_{T}H, version 2.32.

On 16 Jul 1999, 09:18.