Accretion disc reverberation mapping of the quasar 3C 273

TL;DR

This study measures the size of 3C 273's accretion disc using reverberation mapping, finding it larger than theoretical models predict, and explores the disc's structure and its relation to the broad line region.

Contribution

First high-cadence optical reverberation mapping of 3C 273 revealing larger-than-expected accretion disc sizes and supporting a dusty, extended disc model.

Findings

Accretion disc sizes are 2-7 times larger than thin disc theory predicts.

Optical variability is dominated by the accretion disc, not the jet.

Disc size estimates are consistent with the broad line region and dusty outflow models.

Abstract

We present accretion disc size measurements for the well-known quasar 3C 273 using reverberation mapping (RM) performed on high-cadence light-curves in seven optical filters collected with the Las Cumbres Observatory (LCO). Lag estimates obtained using Javelin and PyROA are consistent with each other and yield accretion disc sizes a factor of ~2-7 larger than `thin disc' theoretical expectations. This makes 3C 273 one of a growing number of active galactic nuclei (AGN) to display the so-called `accretion disc size' problem usually observed in low-luminosity AGN. Power-law fits of the form tau~lambda^beta to the lag spectrum, and nufnu ~ nu^beta to the spectral energy distribution (SED) of the variations, both give results consistent with the `thin disc' theoretical expectation of beta=4/3. The Starkey et al. `flat disc with a steep rim' model can fit both the lag estimates and the SED variations. Extrapolating the observed optical lags to putative dust-forming regions of the disc gives r~100-200 light-days. These radii are consistent with the size of the broad line region (BLR) as determined by near-infrared interferometric studies as well as with the best-fit location of the outer edge for the `flat disc with a steep rim' model. Therefore, the accretion disc in 3C 273 might be sufficiently extended to be dusty, allowing the BLR to emerge from it in a dusty outflow. A flux variation gradient analysis and the structure function of our LCO light-curves confirm that the optical variability in 3C 273 is dominated by the accretion disc rather than its radio jet.