Abstract
Centaurus 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 very-high energy gamma ( γ ) ray flux (TeV photons), obtained from H.E.S.S. is relatively faint, hampering detailed light curve analyses in the most energetic energy band. 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 Centaurus A precedes the γ rays by 25 ± 125 days. If this lag is real and related to a γ γ absorption effect in the broad-line region (BLR) around the central source, 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 γ rays and X-rays from Centaurus A.
Highlights
Active galactic nuclei (AGN) are a type of galaxies hosting extremely luminous central regions that can outshine the entire galaxy
We report results of the time-series study using the discrete cross-correlation of light curves as applied to Swift-Burst Alert Telescope (BAT) X-ray and Fermi-Large Area Telescope (LAT) γ-ray long-term observations of the nearest radio galaxy, Centaurus A (Cen A)
The discrete correlation function (DCF) associated with this lag peaked at the value of 0.35 ± 0.14
Summary
Active galactic nuclei (AGN) are a type of galaxies hosting extremely luminous central regions that can outshine the entire galaxy. Two distinct peaks can be seen in such a spectral energy distribution (SED) where the lower energy peak (extending from radio to X-rays) is traditionally accepted as being due to synchrotron emission from relativistic electrons in the jets. Amongst the basic components of the structure of a typical AGN, are the fast-moving high-density clouds surrounding the central engine (but external to the jet) and located closer to the central SMBH than the optically obscuring dust torus. The location of the BLR can be measured by the reverberation method employing the light travel time, the width of the emission line which in turn indicates the speed of the BLR clouds, and the observable temporal lag between the ionizing continuum radiation from the accretion disk and the line emission. We report results of the time-series study using the discrete cross-correlation of light curves as applied to Swift-BAT X-ray and Fermi-LAT γ-ray long-term observations of the nearest radio galaxy, Cen A
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