Estimating AGN Black Hole Masses via Continuum Reverberation Mapping in the Era of LSST

Abstract

Spectroscopic reverberation mapping (RM) is a direct approach widely used to determine the mass of black holes (BHs) in active galactic nuclei (AGNs). However, it is very time consuming and difficult to apply to a large AGN sample. The empirical relation between the broad-line region size and luminosity (Hβ R BLR–L) provides a practical alternative yet is subject to large scatter and systematic bias. Based on the relation between the continuum emitting region size and luminosity (R CER–L) reported by Netzer (2022), we present a new BH mass estimator via continuum RM (CRM) by comparing R CER and R BLR, assuming that the continuum lags are dominated by the diffuse continuum emission. Using a sample of 21 AGNs, we find a tight R BLR–R CER relation (scatter∼0.28 dex) and that R BLR is larger than R CER at 5100 Å by an average factor of 8.1. This tight relation enables the BH mass estimation based on the CRM combined with the velocity information. Applying the relation to rest objects in our CRM sample, we demonstrate that the predicted R BLR,CRM follows the existing Hβ R BLR–L relation well and the estimated CRM BH masses are consistent with the RM/single-epoch BH masses using Hβ. This method will provide significant applications for BH mass estimation thanks to the short continuum lags and the easily accessible high-cadence, large-area photometric data, especially in the era of Legacy Survey of Space and Time.

The Sloan Digital Sky Survey Reverberation Mapping Project: Investigation of Continuum Lag Dependence on Broad-line Contamination and Quasar Properties

“…Lastly, Wang et al (2023) propose that if continuum lag measurements are dominated by diffuse BLR emission, there should be a tight correlation between continuum and BLR lags. Continuum lags dominated by diffuse BLR emission would further imply an R cont -L relation that is analogous to the R BLR -L correlation, notably used to estimate the bulk of black hole mass growth over cosmic time (e.g., Vestergaard & Osmer 2009).…”

“…A relationship between continuum lags and luminosity would be far less observationally demanding than the time-domain spectroscopy required for BLR lag measurements. Wang et al (2023) report a correlation between R BLR and R 5100 , which they further use to imply an R 5100 -L relation, where R 5100 is the continuum size at rest-frame 5100 Å. To investigate a similar correlation in the SDSS-RM sample, we use the subset of 30 quasars that both feature well-measured disk sizes from Homayouni et al (2019) and reliable Hβ BLR size measurements from Grier et al (2017).…”

“…We then convert the observed-frame g-i lag to the rest-frame time delay between the inner accretion disk to the rest-frame 5100 Å emission region [τ 5100 = τ 0 (λ 0 = 5100 Å)]. Similar to Wang et al (2023), we assume a wavelength dependence of β = 4/3 and convert the g-i lag measurements as shown below, following a similar form of Equation (2):…”

“…Figure 7 presents a comparison of the SDSS-RM targets with the result from Wang et al (2023). To accommodate for the upper limits of some of the continuum lag measurements as censored data, we flip the axes between the τ 5100 and Hβ BLR 1 split into a distribution of targets within z con (in red) and outside z con (in blue).…”

“…As shown here, there is possible contamination in RMID 61 in both the g and i bands as they fall within the regions of diffuse UV Fe and Balmer emission, respectively. sizes compared to Wang et al (2023) and convert their best-fit line to the case where τ 5100 is regressed to τ BLR . Among the 30 SDSS-RM quasars with reliable disk size measurements and Hβ BLR measurements, we identify seven quasars with negative continuum lag measurements.…”