Abstract:
Active Galactic Nuclei are powered by matter accreting onto supermassive Balck Holes of 10^7-10^10 solar masses. We know that they reside in almost every galaxy (although often in a dormant phase): their evolution is closely linked to the one of their host galaxies and their feedback is essential to explain the baryon cycle in the Universe.
Rapid variability, especially in the X-ray band, is the most characteristic feature of AGNs reflecting the small size of the emitting region. Long monitoring campaign have been possible
only for very nearby sources. I will show that the recent deep X-ray surveys are revealing variability in all AGNs out to cosmological distances, allowing to link their variability properties to the mass of the central Black Hole and its accretion rates. However the models explaining the variability properties of nearby objects are not consistent with what we observe at high redshift, requiring corrections to current models or the presence of evolutionary effects in the accretion efficiency over cosmic time.
