The behaviour of compact non-thermal sources with pair production

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Abstract
The behaviour of a compact source of hard non-thermal radiation may be strongly influenced by electron–positron pairs created by photon–photon collisions. We investigate the effects on the spectrum and variability of a source when the primary spectrum is due to the Compton scattering of blackbody soft photons by relativistic electrons injected with a fixed high Lorentz factor. Although the model spectra are successful in reproducing the power-law X-ray spectrum common to many Seyfert 1 galaxies, they produce an annihilation feature which is too strong to be acceptable. Many models also produce too many γ-rays, as noted by Lightman & Zdziarski. Further work, relaxing some of our assumptions, may alleviate these problems. The present study attempts to set up the basis from which future work can proceed. Source variability is studied using power-spectral and cross-correlation analyses of the response of a model system to white noise. The characteristic lags which occur between different energy bands in the emergent spectrum are identified. The output light curves show that the light crossing time of the emission regions is significantly less than that obtained from the ‘two-folding’ time-scale, by a factor of at least 2 and more generally a factor of 3 to 7. The compactness parameter of a number of active galaxies is high enough for pair production to be important if they emit γ-rays.

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