X-Ray Emission from Galactic Nuclei
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, similar to the continuum spectra observed in the radio and optical wavebands. The hard X-ray and γ-ray luminosities may exceed the total emission from the nucleus at other wavebands, as is the case with the nearby galaxy Centaurus-A (NGC 5128) in which the spectrum has been confirmed above 1 MeV (Hall et al., 1976). X-ray variability has been observed on timescales of days or less, as discussed by Lawrence, 1979 (see also Elvis et al., 1978). 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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