# The autocorrelation function of Cyg X-1

### Thomas J. Maccarone

### Paolo Coppi

## Abstract

The shot noise variability of Cygnus X-1 has been studied extensively
with frequency domain methods in recent years, but very little has
been done using time domain analysis methods. To complement previous
work, we compute the autocorrelation function of Cygnus X-1 in the
low/hard state in 6 different energy bands with 12 observations from
RXTE. We note most especially that the width of the autocorrelation
function becomes smaller at lower energies, thus ruling out simple
Comptonization models. We compare to simulations of the extended
atmosphere model and show that it produces an autocorrelation which is
wider at higher energies. We discuss the implications of these
results for other models for the variability of Cygnus X-1.

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

On 16 Jul 1999, 09:19.