NAM logo, by David Le Conte

AGN X-ray variability: power spectra, interband correlations, and comparison with XRBs

Rick Edelson (Leicester)

I will review recent observational progress in defining the X-ray variability properties of active galactic nuclei (AGN), made possible by large monitoring campaigns with RXTE and other satellites. These include the first clear evidence for a low-frequency break in the fluctuation power-density spectrum (PDS) that may correspond to a characteristic time scale for the variations. Evidence is also emerging for correlated hard X-ray and EUV variations with the EUV leading by a few hours, but the current evidence for correlations between the X-ray and optical/ultraviolet bands is not very good.

Although these observational advances have yielded some basic constraints, they have not led to major breakthroughs in our understanding of the physical conditions, fueling and acceleration mechanisms in AGN. Pressure for sophisticated theoretical modeling of AGN variability will increase once data start flowing from the new generation of X-ray instruments, most notably XMM, which will be well-suited for further observational progress because of its long uninterrupted viewing window, high throughput, and coaligned optical monitor. Finally, many similarities (and some differences) are emerging in the variability properties of AGN and Galactic X-ray binaries. If a clear link can be established, it may become necessary to invoke a single emission mechanism in objects that differ by more than a factor of 10⁷ in luminosity and mass.

Maintained by Ian Howarth