Probing General Relativity with Accreting Black Holes

Most of the X-ray emission from luminous accreting black holes emerges from within 20 gravitational radii. The effective emission radius is several times smaller if the black hole is rapidly spinning. General Relativistic effects can then be very important. Large spacetime curvature causes strong lightbending and large gravitational redshifts. The hard X-ray, power-law-emitting corona irradiates the accretion disc generating an X-ray reflection component. Atomic features in the reflection spectrum allow gravitational redshifts to be measured. Time delays between observed variations in the power-law and the reflection spectrum (reverberation) enable the physical scale of the reflecting region to be determined. The relative strength of the reflection and power-law continuum depends on light bending. All of these observed effects enable the immediate environment of the black hole where the effects of General Relativity are on display to be probed and explored.Comment: 9 pages, 10 figures, for Proceedings of IAU Symposium 290, Feeding compact objects: Accretion on all scales, eds C.M. Zhang, T. Belloni, M. Mendez & S.N. Zhan

Cooling flows in clusters of galaxies

The gas temperature in the cores of many clusters of galaxies drops inward by about a factor of three or more within the central 100kpc radius. The radiative cooling time drops over the same region from 5 or more Gyr down to about 10^8 yr. Although it would seem that cooling has taken place, XMM and Chandra spectra show no evidence for strong mass cooling rates of gas below 1-2 keV. Chandra images show holes coincident with radio lobes and cold fronts indicating that the core regions are complex. The observational situation is reviewed here and ways in which continued cooling may be hidden are discussed, togther with the implications for any heat source which balances radiative cooling.Comment: To appear in "Lighthouses of the Universe" eds. M. Gilfanov, R. Sunyaev et al., Springer-Verlag; 13 pages, 12 figures

The interaction of radio sources and cooling flows

The X-ray emission in many clusters of galaxies shows a central peak in surface brightness coincident with a drop in temperature. These characterize a cooling flow. There is often a radio source also at the centre of such regions. Data from Chandra now enables us to map the interaction between the radio source and the intracluster medium. Preliminary work shows no sign of heating of the gas beyond the radio lobes, which are often devoid of cooler gas and so appear as holes. In the case of the Perseus cluster around 3C84, the coolest X-ray emitting gas occurs immediately around the inner radio lobes.Comment: 10 pages, 10 figures, Talk at Oxford Radio Galaxies Conference (Aug 2000