Hard X-Ray Lags in Cygnus X-1

We have used the Fourier cross spectra of Cygnus X-1, as obtained with BATSE during a period of almost 2000 days, to estimate the phase (or time) lags between X-ray flux variations in the 20-50 and 50-100 keV bands as a function of Fourier frequency, ν. We find that these lag spectra do not depend on source state, as measured by the fractional rms variations of the X-ray flux. For frequencies well below the Nyquist frequency the variations in the 50-100 keV band lag those in the 20-50 keV band by a time interval τ∝ν−0.8. Binning effects cause the observed hard X-ray time lags to decrease to 0 at the Nyquist frequency. Some previous results on time lags were affected by these binning effects. At photon energies above 10 keV, the time lags are approximately proportional to the logarithm of the ratio the energies of the passbands used.