Abstract
The 2–10 keV X-ray luminosities of galactic nuclei range from Lx ≲ 1036 erg s−1 for our own Galactic nucleus to Lx ≳ 1046 erg s−1 for quasars. The X-ray spectra often appear “hard” and may be considered as quasi-power law continua. The hard X-ray and γ-ray luminosities may exceed the total emission from the nucleus at other wavebands. This is the case with the nearby galaxy Centaurus-A (NGC 5128) and also for 3C 273. In these objects there is firm evidence that the output extends up to γ-ray energies. X-ray variability has been observed in several objects on timescales of a week. (In the low-luminosity Seyfert galaxy NGC 6814, Tennant et al. (1981) report flares on time-scales as short as ∿ 100 seconds.) Such high luminosities, coupled with rapid variations, strongly suggests that the X-rays originate close to the power source in these galactic nuclei.
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Rees, M.J. (1981). Physical Processes for X-Ray Emission in Galactic Nuclei. In: Andresen, R.D. (eds) X-Ray Astronomy. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-9949-3_11
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