Properties of AGN coronae in the NuSTAR era

Understand

The focussing optics of NuSTAR have enabled high signal-to-noise spectra to be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic Black Hole Binaries (BHB).

• Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects.
• Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size.
• Our results indicate that coronae are hot and radiatively compact, lying close to the boundary of the region in the compactness - temperature diagram which is forbidden due to runaway pair production.