Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. XV. Reverberation Mapping of Mg II Emission Lines

TL;DR

This study expands the reverberation mapping of Mg II emission lines in super-Eddington accreting black holes, providing new black hole mass measurements and refining the radius-luminosity relation, revealing shorter Mg II lags at high accretion rates.

Contribution

It presents new Mg II reverberation mapping results for 8 super-Eddington AGNs, updating the Mg II radius-luminosity relation and analyzing the impact of accretion rate on emission line lags.

Findings

Mg II time lags are shorter in high accretion rate AGNs.

Updated the Mg II radius-luminosity relation with a slope of 0.24.

Deviation from the relation correlates with accretion rate.

Abstract

As the 15th paper in a series reporting on a large reverberation mapping (RM) campaign of super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs), we present the results of measurements of the Mg II lines in 18 SEAMBHs monitored spectroscopically from 2017 to 2024. Among these, the time lags of Mg II have been successfully determined for 8 of the 18 objects, thereby expanding the current Mg II RM sample, particularly at higher accretion rates. By incorporating measurements of the line widths, we determine the masses of their central supermassive black holes. Based on these new measurements, we update the relation between the Mg II radius and the monochromatic luminosity at 3000 $\mathring{\mathrm{A}}$ ($R_{\rm MgII}-L_{3000}$ relation), yielding a slope of $0.24 \pm 0.03$, which is slightly shallower than, yet still consistent with, previously reported values. Similar to the H$\beta$ lines, the Mg II time lags in SEAMBHs are shorter than those of AGNs with normal accretion rates at comparable luminosities. The deviation of AGNs from the best-fit $R_{\rm MgII}-L_{3000}$ relation shows a strong correlation with the accretion rate, while no significant correlation is found between the deviation and the flux ratio of UV iron to Mg II.