| Abstract: Accretion on to black holes is the best model to explain the observed radiation from AGN and microquasars. The black-hole microquasars generally show low frequency quasi periodic oscillations (LFQPO) in their hard and intermediate spectral states. LFQPO are oscillations in the hard-powerlaw part of the X-ray spectra, where the oscillation frequencies are in the range of just under 1 Hz to about 20 Hz, but generally around few Hz. LFQPO has been extensively studied. Even relations between QPO frequency and the spectral index of the power law tail has been emperically obtained by numerous observations. But only in a few observations, HFQPO (>50 Hz) has been detected. We present numerical simulation of transonic accretion disc in presence of various dissipative processes to explain LFQPO to HFQPO. In this presentation, we discuss the physics of QPOs in details, why such oscillating regions occur and what are the plasma parameters that governs this. We show that in accretion discs around non-spinning blackholes, oscillations in the inner part of the disc tends to produce LFQPOs. However for highly spinning blackholes, depending on outer boundary conditions, oscillations in the inner part of the disc can produce LFQPOs to HFQPOs. For highly spinning blackhole the oscillating region can be close to the horizon and therefore can produce HFQPO. However, that is a general trend , one can form an oscillating region at a slightly larger distance but can have smaller amplitude and higher frequencies. Due to low statistics there is no emperical relation between spectral index and QPO frequency for HFQPO. We predict a theoretical relation between the two, to be confirmed by more observations in future. |
