Centaurus A: Hard X-ray and soft gamma-ray lightcurve correlation

Abstract

Centaurus A (Cen A), powered by a 55 million solar mass supermassive black hole, has been intensively monitored in all accessible wavelength ranges of the electromagnetic spectrum. However, its gamma-ray flux, both in the H.E.S.S. very-high and the Fermi-LAT high-energy gamma-ray regimes, is relatively faint, hampering lightcurve analyses. Yet, the extensive long-term light curve data from Fermi-LAT and Swift-BAT (hard X-rays) allows for cross-correlation studies. We find a hint that X-ray emission from Cen A precedes the gamma rays by 25 days. If this lag is real and related to a gamma-gamma absorption effect in the broad-line region (BLR) around the central source, then we can constrain the size of the BLR using light-travel time arguments. These are first results of extended light curve correlation studies between high-energy gamma rays and X-rays from Cen A.