Accretion disc reverberation mapping of the quasar 3C 273

ABSTRACT We present accretion disc size measurements for the well-known quasar 3C 273 using reverberation mapping 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 ${\sim} 2{\!-\!}7$ 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 $\tau \propto \lambda ^{\beta }$ to the lag spectrum, and $\nu \, f_\nu \propto \nu ^\beta$ 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 $r\sim 100-200$ 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-fitting 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.