We discuss a model for the γ-ray production in blazars in which electrons are accelerated rectilinearly in localised regions of the jet, scattering soft radiation from the accretion disk. In our model the jet divides naturally into two zones. In the `radiation dominated zone' (close to the disk), the acceleration of electrons is balanced by inverse Compton losses in the Thomson regime and energy is efficiently transferred into the γ-rays. The γ-ray spectral slope is determined by the electric field profile along the jet. In the `particle dominated zone' (further from the disk) the electron losses are too low to balance acceleration and the electrons are injected into the jet with energies corresponding to the full potential drop in the acceleration region. We suggest that these electrons are then isotropised by the random component of the magnetic field of the jet and cool mainly by synchrotron losses. In the framework of our model we predict further that Galactic black hole candidates might be sources of γ-radiation; in this case, however, we do not expect emission above 10GeV.

Spectral formation in steady state, spherical accretion onto neutron stars and black holes is examined by solving numerically and analytically the equation of radiative transfer. The photons escape diffusively and their energy gains come from their scattering off thermal electrons in the converging flow of the accreting gas. We show that the bulk motion of the flow is more efficient in upscattering photons than thermal Comptonization in the range of non-relativistic electron temperatures. The spectrum observed at infinity is a power law with an exponential turnover at energies of order of the electron rest mass. Especially in the case of accretion into a black hole, the spectral energy power-law index is distributed around 1.5. Because bulk motion near the horizon (1-5 Schwarzschild radii) is most likely a necessary characteristic of accretion into a black hole, we claim that observations of an extended power law up to about m_e_c^2^, formed as a result of bulk motion Comptonization, is a real observational evidence for the existence of an underlying black hole.

Supernova 1006 is the first shell type supernova remnant to show evidence of particle acceleration to TeV energies. In the present paper we examine this possibility by modeling the observed X-ray non-thermal emission in terms of synchrotron radiation from Fermi accelerated electrons. The predicted synchrotron spectrum fits the radio and non-thermal component of the observed soft X-ray to hard X-ray emission quite well. These particles can produce TeV gamma rays by inverse Compton scattering on the microwave radiation and other ambient fields, and the derived electron distribution is also used to calculate the expected inverse Compton flux. We find that if the remnant is characterised by a magnetic field strength lower than ~7yG, then the TeV flux can be higher than that of the Crab Nebula. About 75% of the TeV emission from SN 1006 is expected to be concentrated in the synchrotron bright NE and SW rims (the "hard aegis") of the remnant, which would allow a sensitive search if the Atmospheric Imaging Cherenkov Technique is used.

The variable flux of TeV gamma-rays detected from Mkn 421 and Mkn 501 requires the presence of high energy electrons, which could in principle produce large numbers of electron/positron pairs, leading to an electromagnetic cascade. We point out that this scenario can be avoided if electrons are accelerated to high energy rectilinearly, rather than being injected isotropically into a blob, as in most of the models of the GeV gamma-ray emission. By balancing linear acceleration by an electric field against inverse Compton losses in the radiation field of the accretion disk we calculate the emitted spectra and find the conditions which must be fulfilled in order to exclude the development of electromagnetic cascades during acceleration. Assuming these to be fulfilled, we show that the maximum possible photon energy is approximately 10M_8_^2/5^TeV, where M_8_ is the mass of the central black hole in units of 10^8^Msun_. In addition we compute the optical depth to absorption of TeV photons on a possible isotropic scattered component and on the observed nonthermal radiation (in the case of Mkn 421) and find that TeV photons can escape provided the nonthermal X-rays originate in a jet moving with a Lorentz factor γ_b_>8.

We review the hadronic model for Active Galactic Nuclei (AGN). This model, which can be applied to all AGN, advocates the acceleration of protons to ultrarelativistic energies by shock fronts which are formed a few Schwarzschild radii away from the central black hole. The necessary consequences of this hypothesis are discussed. These include the formation of electromagnetic cascades which are initiated by the injection of secondary electrons and photons inside the source, as well as the production and escape of neutrons and neutrinos. As a result of the neutron escape we emphasize that AGN can be sources of TeV radiation.

Shock waves associated with shell type supernova remnants are considered to be possible sites of cosmic ray acceleration. Since shocks are capable of accelerating electrons in addition to protons one anticipates both species to contribute to the high energy radiation expected from these objects. Adopting a simple model for particle acceleration we calculate in a self-consistent manner the time-dependent synchrotron and inverse Compton radiation of high energy electrons assumed either to be accelerated directly by the shock wave or to be injected at high energies as secondaries from the hadronic collisions of relativistic protons with the circumstellar material. We deduce that for standard supernova parameters the TeV flux produced from neutral pion decay is about the same order as the flux expected from directly accelerated electrons.