On the energy dependence of the QPO phenomenon in the black hole system MAXI J1535-571

Abstract

ABSTRACT Previous analysis of AstroSat observations of the black hole system MAXI J1535-571 have revealed the presence of a strong Quasi-Periodic Oscillation (QPO) whose frequency is correlated with the high energy spectral index. Here, we fit the spectra as emitted from a truncated disc with an inner hot corona, study the QPO frequency dependence on other spectral parameters and model the energy-dependent r.m.s and time-lag of the QPO to identify the physical spectral parameters whose variation are responsible for the QPO. The QPO frequency is found to also correlate with the scattering fraction (i.e. the fraction of the soft photons Comptonized) and its dependence on the accretion rate and inner disc radii is consistent with it being the dynamical frequency. The time-lag between the hard and soft photons is negative for QPO frequency >2.2 Hz and is positive for lesser values, making this the second black hole system to show this behaviour after GRS 1915+105. Modelling the energy-dependent time-lag and r.m.s requires correlated variation of the accretion rate, inner disc radii, and the coronal heating rate, with the latter having a time-lag compared to the other two for QPO frequencies less than <2.2 Hz and which changes sign (i.e. the coronal heating variation precedes the accretion rate one) for higher values. The implications of the results are discussed.