The 6.40-6.97 keV Fe Ka lines observed from type 1 Seyfert/quasars are formed by fluorescence in optically-thick nuclear matter that is irradiated by the central X-ray source (e.g. ). Observations by ASCA and XMM-Newton show that the Fe Ka lines are in X-ray bright objects extremely broad with widths corresponding to motion at a substantial fraction of the speed of light (up to about 100 000 km s-1; e.g. [36,37,49]). Such observations allow us to study matter under extreme gravity and to test the predictions from the special and relativistic theory. As predicted by the theory, the lines appear to be skewed towards lower energies, and the overall line profiles suggest an origin in the innermost parts of accretion discs where extreme Doppler/relativistic effects operate. Figure 22.9 shows the most prominent example of a relativistically blurred Fe K line, MCG-6-30-15. The left panel shows the first exciting discovery of a spectrally resolved relativistically blurred Fe K line profile obtained with the ASCA satellite . The highest quality spectrum ever obtained from the Fe K line profile is shown in the right panel of Fig. 22.9 . General relativity effects of gravitationally redshift and special relativity effects of relativistic Doppler boosting effects are clearly visible. The skewed profiles extend below 4 keV which is indicative for a Kerr black hole.
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