Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems

Abstract

Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. By the time the flow reaches the inner edge, the variation in X-rays needs not reflect the true variation of the rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale also to spread around a mean value. In HMXBs, the size of the viscous Keplerian disk is smaller & thus such a spread could be lower as compared to the LMXBs. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a full knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales when there is an ellipticity in the orbit. We study a few compact binaries using long term RXTE/ASM(1.5-12 keV) & Swift/BAT(15-50keV) data to look for such effects & to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation & found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in HMXBs the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in LMXBs with larger Keplerian disk, the peaks are spread out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of the Keplerian disk. Our result confirms different sizes of Keplerian disks in 2 classes of binaries. If the orbital period of a binary system is not known, it may be obtained with reasonable accuracy for HMXBs & with lesser accuracy for LMXBs by our method.