We study Compton upscattering of low-frequency photons in a converging flow of thermal plasma. The photons escape diffusively, and electron scattering is the dominant source of opacity. We solve the equation of radiative transfer in the case of spherical, steady state accretion into black holes numerically and approximately analytically. Unlike previous work on this subject, we consider the inner boundary at a finite radius, and this has a significant effect on the emergent spectrum. It is shown that the bulk motion of the converging flow is more efficient in upscattering photons than thermal Comptonization, provided that the electron temperature in the flow is of order a few keV or less. In this case, the spectrum observed at infinity consists of a soft component coming from input photons that escaped after a few scatterings without any significant energy change and of a power law that extends to high energies and is made of those photons that underwent significant upscattering. The luminosity of the power law is relatively small compared to that of the soft component. The more reflective the inner boundary is, the flatter the power-law spectrum becomes. The spectral energy power-law index for black hole accretion is always higher than 1, and it is approximately 1.5 for high accretion rates. This result tempts us to say that bulk motion Comptonization might be the mechanism behind the power-law spectra seen in black hole X-ray sources.

We show that relativistic bremsstrahlung and inverse Compton scattering of radio-emitting electrons can easily account for the observed γ-ray spectrum of 2EG J1857+0118 if the field strength in the shell is below ~30 μG. This source is located at the eastern border of the composite SNR W44, where the expanding radio shell is interacting with a dense molecular cloud. The nondetection of this remnant above 250 GeV implies a cutoff or steepening in the electron spectrum above ~100 GeV. The E-1.66 spectrum of this radio/γ-ray-emitting electron component is too flat to have its origin in standard first-order Fermi acceleration, but electron injection into the shell by the pulsar PSR B1853+01 over the 2 × 104 yr lifetime may explain why the Crab-like radio spectrum (Sν ~ ν-0.33) is about the hardest of all shell-type remnants. The injected energy would be sufficient to account for the required energy of 6 × 1049 ergs if the initial spin-down power of PSR B1853+01 was about 10 times larger than the present spin-down power of the Crab pulsar. A steeper Fermi electron component may be present, but the observational data are not constraining enough to provide a meaningful limit on the presence of an additional ~E-2 shell-type electron component. The predicted γ-ray contribution from high-energy proton-gas interactions is about 20% of the observed EGRET flux above 100 MeV, which confirms our conclusion that the γ-ray emission from W44 is dominated by a leptonic component.

We present a model of the spectra of gamma-ray emitting blazars in which a single homogeneous emission region both emits synchrotron photons directly and scatters them to high (gamma-ray) energy before emission (a `synchrotron self-Compton' or SSC model). In contrast to previous work, we follow the full time dependent evolution of the electron and photon spectra, assuming a power-law form of the electron injection and examine the predictions of the model with regard to variability of the source. We apply these computations to the object Mkn 421, which displayed rapid variability in its X-ray and TeV emission during a multiwavelength campaign in 1994. This observation strongly implies that the same population of electrons produces the radiation in both energy bands. By fitting first the observed quiescent spectrum over all 18 orders of magnitude in frequency, we show that the time dependence of the keV/TeV flare could have been the result of a sudden increase in the maximum energy of the injected electrons. We show also that different types of flare may occur in this object and others, and that the energy band most sensitive to the properties of the acceleration mechanism is the X-ray band.