On the size of the Fe II emitting region in the AGN Akn 120

TL;DR

This study investigates the size and origin of Fe II emission regions in the AGN Akn 120 using reverberation analysis, finding no clear lag and suggesting the emission may originate from a larger or differently heated region.

Contribution

First reverberation analysis of optical Fe II emission in Akn 120, highlighting the challenges in measuring lag times and discussing possible emission region sizes.

Findings

Fe II emission follows continuum variations over years

No clear reverberation lag detected for Fe II lines

Fe II region likely larger or differently heated than H-beta region

Abstract

We present a reverberation analysis of the strong, variable optical Fe II emission bands in the spectrum of Akn 120, a low-redshift AGN which is one of the best candidates for such a study. On time scales of several years the Fe II line strengths follow the variations in the continuum strength. However, we are unable to measure a clear reverberation lag time for these Fe II lines on any time scale. This is due to the very broad and flat-topped nature of the Fe II cross correlation functions, as compared to the H-beta response which is much more sharply localized in time. Although there is some suggestion in the light curve of a 300-day response time, our statistical analysis does not pick up such a feature. We conclude that the optical Fe II emission does not come from a photoionization-powered region similar in size to the H-beta emitting region, but we cannot say for sure where it does come from. Our results are generally consistent either with emission from a photoionized region several times larger than the H-beta zone, or with emission from gas heated by some other means, perhaps responding only indirectly to the continuum variations.