Date Presented:

2022/07/1

Abstract:

The power-law X-ray spectrum of black-hole X-ray Binaries (BHXRBs) in the hard and hard-intermediate spectral states most probably arises from the interaction of the hot corona with the cold accretion disk. Soft photons from the disk are Comptonized in the corona and some Comptonized photons return to the disk, where they are reprocessed back into soft photons. This system is inherently non-linear as the soft radiation that cools the electrons of the corona is partially produced by the same particle population. We write simple time-dependent equations that describe energy conservation in the system corona-accretion disk and we show that these are of the Lotka-Volterra type. Solving them for constant mass accretion rate, we find that the energy densities of the hot electrons and the Comptonized photons exhibit oscillations that die out after a few cycles. A crucial requirement for the emergence of transient oscillations is that most of the accretion power goes into the corona and the disk is cold. The characteristic frequencies of these oscillations, which depend on the mass accretion rate, the corona size, and the reflection fraction, lie in the range of 1-60 Hz for typical parameter values. We also demonstrate that the natural frequencies persist even in the case of variable accretion rates. We make, therefore, a tentative claim that type-C Quasi Periodic Oscillations in the hard state of BHXRBs could, in principle, arise from the interaction of the hot Comptonizing corona with the much colder accretion disk.

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