Swift/UVOT grism monitoring of NGC 5548 in 2013: an attempt at MgII reverberation mapping

TL;DR

This study attempted to perform reverberation mapping of the MgII emission line in NGC 5548 using Swift UV observations, but found no significant lag, questioning the use of MgII for black hole mass estimates at high redshift.

Contribution

First direct reverberation mapping attempt of MgII in NGC 5548, highlighting challenges in using MgII for black hole mass scaling relations.

Findings

MgII emission shows weak correlation with continuum variability.

No significant time lag detected between MgII and continuum.

Results cast doubt on MgII-based black hole mass estimates at high redshift.

Abstract

Reverberation-mapping-based scaling relations are often used to estimate the masses of black holes from single-epoch spectra of AGN. While the radius-luminosity relation that is the basis of these scaling relations is determined using reverberation mapping of the H$\beta$ line in nearby AGN, the scaling relations are often extended to use other broad emission lines, such as MgII, in order to get black hole masses at higher redshifts when H$\beta$ is redshifted out of the optical waveband. However, there is no radius-luminosity relation determined directly from MgII. Here, we present an attempt to perform reverberation mapping using MgII in the well-studied nearby Seyfert 1, NGC 5548. We used Swift to obtain UV grism spectra of NGC 5548 once every two days from April to September 2013. Concurrent photometric UV monitoring with Swift provides a well determined continuum lightcurve that shows strong variability. The MgII emission line, however, is not strongly correlated with the continuum variability, and there is no significant lag between the two. We discuss these results in the context of using MgII scaling relations to estimate high-redshift black hole masses.