Continuum Reverberation Mapping of the Accretion Disks in Two Seyfert 1 Galaxies

TL;DR

This study measures optical continuum lags in two Seyfert 1 galaxies, finding larger-than-expected accretion disk sizes that suggest higher accretion rates, with implications for disk models and X-ray reprocessing theories.

Contribution

First optical continuum lag measurements for two Seyfert 1 galaxies using multi-band monitoring, testing accretion disk models and revealing larger disk sizes than standard theory predicts.

Findings

Lags consistent with thin-disk models but larger than predictions.

Observed lags are 2-3 times bigger than standard thin-disk theory.

X-ray variability precedes UV/optical variability, challenging reprocessing models.

Abstract

We present optical continuum lags for two Seyfert 1 galaxies, MCG+08-11-011 and NGC 2617, using monitoring data from a reverberation mapping campaign carried out in 2014. Our light curves span the ugriz filters over four months, with median cadences of 1.0 and 0.6 days for MCG+08-11-011 and NGC\,2617, respectively, combined with roughly daily X-ray and near-UV data from Swift for NGC 2617. We find lags consistent with geometrically thin accretion-disk models that predict a lag-wavelength relation of $\tau \propto \lambda^{4/3}$. However, the observed lags are larger than predictions based on standard thin-disk theory by factors of 3.3 for MCG+08-11-011 and 2.3 for NGC\,2617. These differences can be explained if the mass accretion rates are larger than inferred from the optical luminosity by a factor of 4.3 in MCG+08-11-011 and a factor of 1.3 in NGC\,2617, although uncertainty in the SMBH masses determines the significance of this result. While the X-ray variability in NGC\,2617 precedes the UV/optical variability, the long 2.6 day lag is problematic for coronal reprocessing models.