Coronal cooling and its signatures in the rapid aperiodic variability of Galactic black-hole candidates

The most popular models for the complex phase and time lags in the rapid aperiodic variability of Galactic X-ray binaries are based 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 disc). However, in their original version, these models have neglected the additional cooling of the coronal plasma due to 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 even be 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 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 disc 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.Comment: 23 pages, including 8 figures and 2 tables; accepted for publication in ApJ; extended discussion w.r.t. original versio

Pair annihilation radiation from relativistic jets in gamma-ray blazars

The contribution of the pair annihilation process in relativistic electron-positron jets to the gamma-ray emission of blazars is calculated. Under the same assumptions as for the calculation of the yield of inverse Compton scattered accretion disk radiation (Dermer and Schlickeiser 1993) we calculate the emerging pair annihilation radiation taking into account all spectral broadening effects due to the energy spectra of the annihilating particles and the bulk motion of the jet. It is shown that the time-integrated pair annihilation spectrum appears almost like the well-known gamma-ray spectrum from decaying $\pi^o$-mesons at rest, yielding a broad bumpy feature located between 50 and 100 MeV. We also demonstrate that for pair densities $> 10^9$ cm$^{-3}$ in the jet the annihilation radiation will dominate the inverse Compton radiation, and indeed may explain reported spectral bumps at MeV energies. The refined treatment of the inverse Compton radiation leads to spectral breaks of the inverse Compton emission in the MeV energy range with a change in spectral index $\Delta \alpha$ larger than 0.5 as detected in PKS 0528+134 and 3C273