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Extensive monitoring is now available for a good sample of the brighter quasars and BL Lacs. In some cases, lightcurves are well enough sampled to permit stochastic modelling. The authors present here, a simple model of the lightcurve of the prototype BL Lacertae object, BL Lac in terms of a series of random events, each with rapid rise and

With an adequate supply of matter, the inward drifting of dust particles induced by the Poynting-Robertson effect can lead to the formation of a shell or disk-like structure around the source of radiation. In a steady state, the Poynting-Robertson effect produces a spatial density of dust in the form of 1/r which in turn gives rise to a mid

Contents: 1. Introduction. 2. The virial equation. 3. Components of kinetic energy. 4. Gravitational potential energy. 5. Validity of putting r.h.s. terms zero. 6. A new relationship. 7. The early universe. 8. Estimates of Ω0 and neutrino mass. 9. Discussion and conclusion. Appendix: The virial equation for massive and massless particles in any set

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First order Fermi acceleration is an efficient mechanism to produce relativistic particles in strong astrophysical shocks. The authors have developed a model using this theory to explain the non-thermal emission from non-relativistic jets in quasars and AGNs.