The aperiodic variability of Cyg X-1 and GRS 1915+105 at very low frequencies

We have carried out a timing analysis of Cyg X-1 and GRS 1915+105 using more than 5 years worth of data from the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE). We have obtained for the first time power density spectra, colour-colour and colour-intensity plots, cross-correlation functions and phase-lag diagrams to investigate the variability of the sources at frequencies <10(-5) Hz. We find that the power spectra are not at but consistent with a power-law of index ∼-1. This is the same slope as that found in the power spectra of Cyg X-1 during the soft state. In fact, the power spectrum of Cyg X{1 in the frequency range 10(-7)-10(-5) Hz appears as a continuation of the 10(-3)-10 Hz power spectrum during the soft state. Significant variability, expressed as the fractional rms in the frequency range 2 x 10(-7)-1 x 10(-5) Hz, is detected at a level of 27% and 21% for GRS 1915+105 and Cyg X-1, respectively. Our results confirm previous suggestions that the innermost regions of the accretion disc, where the X-ray photons are produced, are affected by variations occurring at larger radii from the black hole, presumably due to changes in the mass accretion rate. We also observe strong spectral changes: in Cyg X-1 the spectrum softens as the flux increases when it is in the low/hard state, and hardens with increasing flux in the high/soft state. GRS 1915+105 follows the same trend as Cyg X-1 in the soft state, but the hardness ratios show a larger amplitude of variation. The cross-correlation shows that the variations occur simultaneously in all energy bands. However, the cross-correlation functions are asymmetric toward negative lags. The phase spectrum also reveals negative lags at periods larger than a few days, suggesting that the long term variations in the soft energy band are delayed with respect to the harder bands.