Coronal Cooling and Its Signatures in the Rapid Aperiodic Variability of Galactic Black Hole Candidates

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Abstract
The most popular models for the complex phase and time lags in the rapid aperiodic variability of Galactic X-ray binaries are based on the Comptonization of soft seed photons in a hot corona, where small-scale flares are induced by flares of the soft seed photon input (presumably from a cold accretion disk). However, in their original version, these models have neglected the additional cooling of the coronal plasma as a result of the increased soft seed photon input and assumed a static coronal temperature structure. In this paper, our Monte Carlo/Fokker-Planck code for time-dependent radiation transfer and electron energetics is used to simulate the self-consistent coronal response to the various flaring scenarios that have been suggested to explain phase and time lags observed in some Galactic X-ray binaries. It is found that the predictions of models involving slab-coronal geometries are drastically different from those deduced under the assumption of a static corona. However, with the inclusion of coronal cooling they may be even more successful than in their original version in explaining some of the observed phase and time lag features. The predictions of the model of inward-drifting density perturbations in an advection-dominated accretion flow (ADAF)-like, two-temperature flow also differ from the static corona case previously investigated, but may be consistent with the alternating phase lags seen in GRS 1915+105 and XTE J1550-564. Models based on flares of a cool disk around a hot, inner two-temperature flow may be ruled out for most objects where significant Fourier frequency-dependent phase and time lags have been observed.