Robotic reverberation mapping of the broad-line radio galaxy 3C 120

TL;DR

This study used reverberation mapping with robotic telescopes to measure the black hole mass and accretion disc size in the galaxy 3C 120, providing precise results consistent with previous findings and revealing larger-than-expected disc sizes.

Contribution

First detailed reverberation mapping of 3C 120 using robotic telescopes, measuring black hole mass and accretion disc size with improved accuracy.

Findings

Black hole mass of approximately 6.3 x 10^7 solar masses.

Continuum lags consistent with thin disc model but larger than standard predictions.

Velocity-resolved lags show symmetric pattern across Hβ line.

Abstract

We carried out photometric and spectroscopic observations of the well-studied broad-line radio galaxy 3C 120 with the Las Cumbres Observatory (LCO) global robotic telescope network from 2016 December to 2018 April as part of the LCO AGN Key Project on Reverberation Mapping of Accretion Flows. Here, we present both spectroscopic and photometric reverberation mapping results. We used the interpolated cross-correlation function (ICCF) to perform multiple-line lag measurements in 3C 120. We find the H$\gamma$, He II $\lambda 4686$, H$\beta$ and He I $\lambda 5876$ lags of $\tau_{\text{cen}} = 18.8_{-1.0}^{+1.3}$, $2.7_{-0.8}^{+0.7}$, $21.2_{-1.0}^{+1.6}$, and $16.9_{-1.1}^{+0.9}$ days respectively, relative to the V-band continuum. Using the measured lag and rms velocity width of the H$\beta$ emission line, we determine the mass of the black hole for 3C 120 to be $M=\left(6.3^{+0.5}_{-0.3}\right)\times10^7\,(f/5.5)$ M$_\odot$. Our black hole mass measurement is consistent with similar previous studies on 3C 120, but with small uncertainties. In addition, velocity-resolved lags in 3C 120 show a symmetric pattern across the H$\beta$ line, 25 days at line centre decreasing to 17 days in the line wings at $\pm4000$ km s$^{-1}$. We also investigate the inter-band continuum lags in 3C 120 and find that they are generally consistent with $\tau\propto\lambda^{4/3}$ as predicted from a geometrically-thin, optically-thick accretion disc. From the continuum lags, we measure the best fit value $\tau_{\rm 0} = 3.5\pm 0.2$ days at $\lambda_{\rm 0} = 5477$A. It implies a disc size a factor of $1.6$ times larger than prediction from the standard disc model with $L/L_{\rm Edd} = 0.4$. This is consistent with previous studies in which larger than expected disc sizes were measured.