Revisiting the H$\beta$ Size-Luminosity Relation Using a Uniform Reverberation-Mapping Analysis

TL;DR

This study refines the Hβ size-luminosity relation for AGNs using a uniform reverberation-mapping approach, revealing a flatter slope and systematic differences based on luminosity tracers and Eddington ratios.

Contribution

It provides a consistent analysis of AGN BLR sizes with multiple lag measurement methods and compares various luminosity tracers, improving the understanding of the size-luminosity relation.

Findings

ICCF is the most reliable lag measurement method.

The BLR size-luminosity relation has a flatter slope than 0.5.

Systematic offsets are observed in different luminosity tracers for different Eddington ratios.

Abstract

We revisit the relation between active galactic nucleus (AGN) broad-line region (BLR) size and luminosity by conducting a uniform H$\beta$ reverberation-mapping analysis for 212 AGNs with archival light curves. Our analysis incorporates three different lag measurement methods, including the interpolated cross-correlation function (ICCF), JAVELIN, and PyROA, alongside a consistently defined lag searching window and an alias removal procedure. We find that ICCF, albeit with larger uncertainties compared to other methods, is the most reliable method based on our visual inspection of the matches between H$\beta$ and the shifted continuum light curves. Combining this sample with the 32 AGNs from the Seoul National University AGN Monitoring Project, we obtain the best-fit relation between the BLR size ($R_{\rm BLR}$) and the continuum luminosity at 5100$\r{A}$ ($L_{5100}$) with a slope significantly flatter than 0.5. By selecting a subsample of 157 AGNs with the best-quality lag measurements using a set of quantitative criteria and visual inspection, we find a consistent slope and a slightly decreased intrinsic scatter. We further investigate the effect of luminosity tracers, including $L_{5100}$, H$\beta$ luminosity ($L_{\rm H\beta}$), ${\rm [O\,\mathrm{III}]}$ luminosity ($L_{\rm [O\,\mathrm{III}]}$), and 2 to 10 keV hard X-ray luminosity ($L_{\rm 2\unicode{x2013}10\,keV}$). We find that sub-Eddington and super-Eddington AGNs exhibit systematic offsets in both $R_{\rm BLR}$$\unicode{x2013}$$L_{5100}$ and $R_{\rm BLR}$$\unicode{x2013}$$L_{\rm H\beta}$ relation, but are comparable in the $R_{\rm BLR}$$\unicode{x2013}$$L_{\rm [O\,\mathrm{III}]}$ and $R_{\rm BLR}$$\unicode{x2013}$$L_{\rm 2\unicode{x2013}10\,keV}$ relation. We discuss the potential causes for these different deviations when employing different luminosity tracers.