ACTIVE galactic nuclei (AGNs) exhibit high luminosity with rapid variability, especially in X-ray emission, for which the luminosity can be greater than that of a normal galaxy, and the variability timescale implies an emitting region in some cases smaller than one light hour1. The hard X-ray spectrum of AGNs is nonthermal, probably arising from an electron-positron pair cascade, with some emission reflected off relatively cold matter2,3. Energy can be pumped into a cascade by any process that produces relativistic pairs, but because electrons and positrons are hard to accelerate efficiently, there has been interest in models in which protons are accelerated4,5, and create relativistic electrons on interaction with a local radiation field6-8. Here we show that a sufficient column density of protons can lead to runaway pair production: photons generated by the relativistic pairs are the targets for the protons to produce more pairs. This process can produce X-ray flares with the observed characteristics, and our model predicts the maximum ratio of luminosity to source size ('compactness') as well as their spectrum in the early phases. The same mechanism may also be able to create the knots of synchrotron-radiating pair plasma seen in sources such as 3C273.

When energetic electrons Compton scatter soft photons in the Klein-Nishina regime, the produced gamma rays are subject to photon-photon pair attenuation, and a pair-Compton cascade results. We examine the formation of such cascades in the spatially varying photon fields encountered by high-energy electrons and photons emitted from luminous compact objects. Constraints on the geometry of the soft photon source and the location of the gamma-ray production site are deduced from observations of 100 MeV and TeV gamma rays emitted by galactic compact objects and active galactic nuclei.

Existing models for X-ray emission from novae in outburst encounter several difficulties. The X-ray flux that may be expected to result from Compton degradation of gamma rays produced by radioactive decays is calculated. It is demonstrated that radioactive decays of Na-22 may play an interesting role in the production of hard X-rays, particularly in novae enriched in oxygen, neon, and magnesium.

We propose that the important relationship between 3C 273 and 3C 279, the first two extragalactic sources detected at > 100 MeV energies, is their superluminal nature. In support of this conjecture, we propose a kinematic focusing mechanism, based on Compton scattering of accretion-disk photons by relativistic nonthermal electrons in the jet, that preferentially emits gamma rays in the superluminal direction.

Compton scattering of low-frequency photons in a converging flow of cold plasma is studied. The equation of radiative transfer in the case of spherical near-critical steady state accretion onto a neutron star is solved analytically. The inner boundary condition is that the neutron star surface is completely reflective, or that there is a magnetopause with an empty cavity inside it. The photons escape diffusively and electron scattering is the dominant source of opacity. The energy gain of the photon comes entirely from the bulk motion of the converging flow of the accreting gas. The spectrum observed at infinity is a power law at high frequencies with photon number spectral index essentially -1. This spectrum is significantly flatter than that found for accretion into black holes.