Accretion onto compact objects such as black holes and neutron stars is the most efficient method of releasing energy, with up to 40% of the rest-mass energy of the matter accreting on the black hole able to be liberated.
High luminosities due to accretion are observed in X-ray binaries, where stellar mass black holes accrete mass from a companion star and in active galactic nuclei (AGN) where accretion onto a supermassive black hole in the central region is observed to outshine all the billions of stars in the galaxy itself.
In addition to the thermal emission detected from the accretion disc, the X-ray spectrum of an accreting black hole is often dominated by a power law with a high energy rollover, thought to originate from inverse Compton scattering of lower energy photons from the accretion disc by high energy electrons in a hot corona surrounding the central black hole.
These high energy X-rays will also be reflected from the accretion disc, imprinting a number of emission lines and absorption features on the observed spectrum. If this reflection originates from close to the black hole, the energy of observed emission lines will be affected by gravitational redshift, with emission from closer to the black hole being shifted to lower energies. Studying the profile and position of emission lines in observed spectra can therefore trace the material around the black hole and even infer the black hole’s spin.
Further constraints on the structure of the central regions of the black hole can be obtained by studying the variability of the emission. For example, the reflection spectrum changes in response to changes in the illuminating radiation in a process called reverberation. The time lags between the continuum and reflected spectral component can give us an upper limit on the distance of their origins.
All observational results can be understood in the context of theoretical models produced by, for example, raytracing simulations of the X-ray emission around the central black hole or MHD simulations of accretion discs.