Spectra and Variability of X-ray emission from accreting black holes

Studying X-Ray spectra and timing of emission from accreting black holes.

As matter falls onto black holes, it heats up and emits high energy radiation. Studying the precise nature of this radiation allows inferences to be made about the nature of the structure around the black hole.

It has been learnt that an accretion disc forms which typically emits in UV for the supermassive black holes at the centre of galaxies and in soft X-rays for stellar mass black holes.

Some of this emission is then scattered to X-ray wavelengths by a population of hot electrons, known as a corona, whose exact nature is unknown.

New high-energy observatories, in particular NuSTAR, are able to take spectra of the emission from this corona up to its high-energy cut-off, providing information about important properties such as the temperature of the electrons.

The accretion disc also provides a surface off of which photons can scatter, forming atomic reflection features. How atomic lines are shifted by general relativistic effects near the central black hole can provide information about the inner accretion disc and the spacetime in which it resides.

As the emission leaves the black hole to reach us, it passes through clouds of hot gas, which imprint absorption features which may be used to determine the structure of the gas clouds around the black hole.

The emission is also strongly variable and studying how different bands vary with respect to each other can provide information on their relationship and geometry.

When coronal emission is reflected from the disc, changes in the reflected components are expected to show a lag relative to the direct coronal emission. The size of this lag sets an upper bound on the distance to the reflecting region of the disc, given by the distance light can travel in this time.

Combining these various sources of information will allow further details of accretion around black holes to be elucidated.