Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-01-31
Astronomy and Astrophysics
Astrophysics
4 pages, 3 colour figures. To appear in the Proceedings of the Symposium on `New Visions of the X-ray Universe in the XMM-Newt
Scientific paper
We present a model for magnetic structured coronae above accretion discs. On the shortest timescales, spatially and temporally correlated coronal flares can explain X-ray temporal and spectral variability observed in Seyfert galaxies. In particular, power density spectra, flux-spectral index and flux-variance correlations are naturally accounted for by the model. More dramatic spectral variations (i.e. state transitions in GBHC) are associated with parameters varying on longer timescales, such as accretion rate, coronal strength or geometry of the inner disc. In the framework of the standard Shakura--Sunyaev accretion disc theory, here we discuss why energetically dominant coronae at low accretion rates are ideal sites for launching powerful MHD driven outflows. Then, if the outflow is radiatively inefficient, then so is the source overall, even without advection being relevant for the dynamics of the accretion flow. This could be an alternative scenario for LLAGN and GBHC in their low/hard state, and may have consequences for our understanding of the accretion history of the universe.
Fabian Andrea C.
Merloni Andrea
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