Gamma-Rays from Blazars: a Comparison of 3C 279, PKS 0537-441 and Mrk 421

Summary

Abstract

The γ-ray emission from three of the sources detected by the COMPTON Observatory is discussed in the framework of relativistic jets emitting via the synchrotron self-Compton mechanism. The physical conditions in the three sources are derived and compared.

One of the most exciting discoveries of the COMPTON Observatory is the observation of γ-ray emission from 16 blazars. The broad band continuum (radio to X-ray) from these objects can be understood on the basis of the synchrotron self-Compton process in an inhomogeneous jet of plasma moving with relativistic speed (e.g.). In this model the synchrotron emission usually extends up to the UV and possibly to the X-ray bands. It is therefore natural to expect γ-rays produced by first order self-Compton scattering. The strength of this component depends on the ratio between the radiation and magnetic energy densities, and on the optical depth to photon-photon absorption. The latter process can be avoided if the source is beamed, and independent evidence for beaming is indeed found from radio and X-ray observations.

Nevertheless the γ-ray intensity measured from 3C 279 was a great surprise, in that the γ-ray power exceeded that in all other bands. This requires the radiation energy density to be larger than the magnetic energy density, a situation usually called the “Compton catastrophe” since at first sight it gives rise to divergent Compton energy losses.