@article {22757,
	title = {X-ray flares from runaway pair production in active galactic nuclei},
	volume = {360},
	year = {1992},
	month = {1992/11/1},
	pages = {135 - 137},
	abstract = {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 hour<SUP>1</SUP>. The hard X-ray spectrum of AGNs is nonthermal, probably arising from an electron-positron pair cascade, with some emission reflected off relatively cold matter<SUP>2,3</SUP>. 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 accelerated<SUP>4,5</SUP>, and create relativistic electrons on interaction with a local radiation field<SUP>6-8</SUP>. 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 ({\textquoteright}compactness{\textquoteright}) 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.},
	keywords = {active galactic nuclei, Astrophysics, Electron-Positron Pairs, Luminosity, X Ray Sources, X Ray Spectra},
	isbn = {0028-0836},
	url = {https://ui.adsabs.harvard.edu/abs/1992Natur.360..135K},
	author = {Kirk, J. G. and Mastichiadis, A.}
}
