Fast spectral variability from Cygnus X-1

Abstract

We have developed an algorithm that, starting with the observed properties of the X-ray spectrum and fast variability of an X-ray binary, allows the production of synthetic data reproducing observables such as power density spectra and time lags as well as their energy dependence. This further allows us to reconstruct the variability of the parameters of the energy spectrum and to reduce substantially the effects of Poisson noise, helping in studying fast spectral variations. We have applied the algorithm to the Rossi X-ray Timing Explorer data of the black hole binary Cygnus X-1, fitting the energy spectrum with a simplified power-law model. We found that the distribution of the power-law spectral indices on time-scales as low as 62 ms is limited between 1.6 and 1.8. The spectra index is positively correlated with the flux even on such time-scales. © 2010 The Authors. Journal compilation © 2010 RAS.