The luminosity-dependent contribution from the broad line region to the wavelength-dependent lags in Mrk 110

TL;DR

This study measures wavelength-dependent lags in Mrk 110, revealing that the broad line region's contribution to these lags varies with X-ray luminosity, primarily due to changes in the BLR's inner radius.

Contribution

It demonstrates that the BLR's contribution to wavelength-dependent lags in AGN varies with luminosity, highlighting the role of the BLR's inner radius in this process.

Findings

Excess lags appear at higher X-ray luminosities.

Lag changes correlate with the BLR's inner radius adjustments.

Reprocessing lags from the accretion disc remain nearly constant.

Abstract

We have measured the wavelength-dependent lags between the X-ray, UV and optical bands in the high accretion rate ($L/L_{\rm Edd}\approx40\%$) Active Galactic Nucleus Mrk 110 during two intensive monitoring campaigns in February and September 2019. We divide the observations into three intervals with different X-ray luminosities. The first interval, already published in Vincentelli et al. (2021), has the lowest X-ray luminosity and did not exhibit the U-band excess positive lag, or the X-ray excess negative lag that is seen in most AGN. However, these excess lags are seen in the two subsequent intervals of higher X-ray luminosity. Although the data are limited, the excess lags appear to scale with X-ray luminosity. Our modelling shows that lags expected from reprocessing of X-rays by the accretion disc vary hardly at all with increasing luminosity. Therefore, as the U-band excess almost certainly arises from Balmer continuum emission from the broad line region (BLR), we attribute these lag changes to changes in the contribution from the BLR. The change is easily explained by the usual increase in the inner radius of the BLR with increasing ionising luminosity.