How Are the Outbursts Around Black Holes Triggered?

Abstract

We study accretion processes under two component accretion flow paradigm where an accretion flow has Keplerian and sub-Keplerian components. Here, observational studies carried out by authors in earlier studies using this approach are revisited. Using various methods, Shakura and Sunyaev’s viscosity parameter, α values in both sub-Keplerian and Keplerian components are obtained during outbursts of various sources. The values of α were found to be supercritical in Keplerian component and subcritical in sub-Keplerian component always (as suggested in previous studies). Subsequently, a study of variation of sub-Keplerian values of α was then carried out which confirms that this value rose gradually in rising phase and decreased in declining phase for all outbursting sources. Spectral state transitions took place for some particular limits of sub-Keplerian values of α. For its values less than 0.008, all sources were categorized in hard spectral state. When it was in the ranges 0.008–0.035 and 0.035–0.01, hard and soft intermediate states were observed respectively. Soft spectral state would be expected for any larger values of α in the sub-Keplerian component. Thus the concept of viscosity enhancement being responsible for triggering the outbursts and spectral state changes around black holes has been strengthened. A study with more outbursts of larger number of sources should be carried out to confirm this value of α which triggers these outbursts and spectral state transitions, predict various line parameters relevant for their future astronomical detection.