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

TL;DR

This work extends Mg II reverberation mapping to eighteen high-accretion-rate AGNs, obtaining eight robust Mg II lags and using line widths to estimate SMBH masses in the range $8.1\lesssim\log(M_{ullet}/M_{\ m odot})\lesssim8.7$. By combining these measurements with previous Mg II RM samples, the authors derive a refined $R_{\rm MgII}-L_{3000}$ relation with slope $0.24\pm0.03$ and intercept $2.05\pm0.02$, plus an intrinsic scatter of $0.04$ and total scatter of $0.20$. They demonstrate that SEAMBHs show systematically shortened Mg II lags compared with AGNs of normal accretion, with the offset correlating strongly with the dimensionless accretion rate $\dot{\mathscr{M}}$, while the UV iron strength $\mathcal{R}_{\rm Fe}^{\rm UV}$ shows no significant link to the lag residuals. The results support accretion-rate–dependent BLR structure and self-shadowing in slim disks as a mechanism for shorter lags, and they pave the way for more precise Mg II–based SMBH mass estimates at higher redshifts. Overall, the work strengthens the Mg II RM framework as a tool for probing SMBH growth in the distant universe and clarifies the role of accretion physics in shaping BLR scales.

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$β$ 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.

Figures (48)

• Figure 1: Mean and rms spectra. The blue, purple, and red spectra in each panel represent the mean spectra, rms spectra, and rms spectra generated from the residuals after subtracting the power law and iron template (see more details in Section \ref{['sec_lightcurve']}), respectively. The spectra are plotted with rest-frame wavelength versus Galactic-extinction-corrected flux in the observed frame, consistent with the subsequent figures. The gray lines represent the error spectra for comparison. The gray shadow marks the 2775--2825 Å integration window for measurements of Mg ii light curves. The tick labels on the left and right y axes correspond to the mean and rms spectra, respectively. All spectra are in units of $10^{-16} \, {\rm erg} \, {\rm s}^{-1} \, {\rm cm}^{-2} \, \text{\AA}^{-1}$. The signal-to-noise ratio (SNR) at 3000Å of the mean spectrum is listed at each panel.
• Figure 2: An example of the spectral fitting results. The black line represents a single epoch spectrum of J091245 in the rest frame after correcting for Galactic extinction. The signal-to-noise ratio (SNR) of this spectrum is explicitly written. The red line indicates the best-fit model. The orange, green, and blue lines correspond to the power law, iron template, and Mg ii components, respectively (see Section \ref{['sec_lightcurve']}).
• Figure 3: Light curves and time-lag measurements. The left panels show the 3000 Å continuum light curves after intercalibration alongside the Mg ii light curves. "LJ/CAHA", "LJ/CAHA Phot", and "ZTF" represent the Lijiang/CAHA spectroscopic data, Lijiang/CAHA photometric data, and ZTF photometric data points, respectively. The continuum light curves are in units of $10^{-16} \, {\rm erg} \, {\rm s}^{-1} \, {\rm cm}^{-2} \, \text{\AA}^{-1}$, while the Mg ii light curves are in units of $10^{-15} \, {\rm erg} \, {\rm s}^{-1} \, {\rm cm}^{-2}$. The gray shaded regions represent the reconstructions from the MICA model, incorporating additional systematic errors that were automatically included during the reconstruction process. The upper-right panel displays the auto-correlation function (ACF) of the continuum light curve. The lower-right panel shows the ICCF (black line), CCCD (blue histogram), and time-lag distribution obtained from MICA (orange histogram) in the observed frame. The complete figure set (8 images) is available in the online article.
• Figure 3: (Continued.)
• Figure 3: (Continued.)