Time Domain Analysis of Variability in Cygnus X-1: Constraints on the Emission Models

We use time domain analysis techniques to investigate the rapid variabilityof Cygnus X-1. We show that the cross-correlation functions between hard andsoft energy bands reach values very close to unity and peak at a lag of lessthan 2 millisecond for energies separated by a factor of 10. This confirms thatthe process that produces X-ray photons at different energies is extremelycoherent on short time scales and strongly constrains emission models proposedto explain Fourier-frequency-dependent time lags. We present autocorrelationfunctions at different energies, and note their widths decrease with increasingenergy. We show that the extended Compton corona model producesauto-correlation functions whose widths increase with increasing energy, thatthe model of cylindrical waves moving inward through a transition disk has toolarge a peak lag in the cross-correlation function. Models of magnetic flaringand of drifting blobs in a hot corona can qualitatively fit the observations.Comment: 4 pages, 3 figures, accepted to Astrophysical Journal Letter