Relativistic accretion disks in AGN

AGN are believed to contain a massive black hole surrounded by an accretion disk. Despite the recent successes in imaging the inner regions of AGNs (notably using the Hubble Space Telescope), it is still not possible to take a close-up picture of their inner regions. Scientists believe that the X-ray emission line data obtained with ASCA have proven the presence of a relativistic accretion disk.

Emission line photons have a specific energy (or wavelength, or frequency), dictated by the atomic physics, as seen from the matter that emitted them. When the emitting matter is moving, then we detect the photons at Doppler shifted energies. In an accretion disk, material near the outside moves slower than material in the inner parts. Depending on the angle at which we observe the accretion disk, and ignoring relativistic effects for now, emission lines from an accretion disk have a characteristic double-horned structure.

A schematic diagram of the origin of double-horned structure, for an accretion disk in a binary. Top: Gas in each zone of the disk is coming toward, or receding from, us with a similar velocity (they have very different sideways motion but that does not matter for Doppler shifts). Bottom: Adding up contribution of all the gas in each zone, we can calculate the emission line profile --- the result is a characteristic double-horned shape.

When the accretion disk surrounds a black hole, however, the relativistic effect must also be considered. Photons coming from close to the black hole (those that are also at extreme wings in the non-relativistic case above) are gravitationally redshifted, introducing a characteristic distortion. This is exactly the model that fits the iron emission line data on several AGNs observed with ASCA, first demonstrated in a four-day observation of the Seyfert galaxy MCG-6-30-15, as shown below.

This result is a convincing demonstration of a relativistic accretion disk surrounding a massive black hole. Indirectly, but surely, we are observing the regions very close to the central black hole with ASCA data. Moreover, with further high-quality data, we can hope to constrain our viewing geometry and whether (or how fast) the central black hole is rotating.