Long term optical and X-ray variability of the Seyfert galaxy Markarian 79

TL;DR

This study analyzes five years of concurrent X-ray and optical data of Seyfert galaxy Mrk 79, revealing short-term correlated variability likely due to X-ray reprocessing and long-term optical variations driven by accretion rate changes or corona position shifts.

Contribution

It provides a detailed multi-year analysis of optical and X-ray variability in Mrk 79, highlighting the different origins of short-term and long-term variations.

Findings

Short-term optical and X-ray fluxes are significantly correlated with negligible delay.

Short-term optical variations are consistent with X-ray reprocessing in the accretion disc.

Long-term optical variations are driven by independent mechanisms such as accretion rate changes or corona position shifts.

Abstract

We present the results of concurrent X-ray and optical monitoring of the Seyfert 1 galaxy Mrk 79 over a period of more than five years. We find that on short to medium time-scales (days to a few tens of days) the 2-10 keV X-ray and optical u and V band fluxes are significantly correlated, with a delay between the bands consistent with zero days. We show that most of these variations may be well reproduced by a model where the short-term optical variations originate from reprocessing of X-rays by an optically thick accretion disc. The optical light curves, however, also display long time-scale variations over thousands of days, which are not present in the X-ray light curve. These optical variations must originate from an independent variability mechanism and we show that they can be produced by variations in the (geometrically) thin disc accretion rate as well as by varying reprocessed fractions through changes in the location of the X-ray corona.