Intensive Swift and LCO monitoring of PG 1302$-$102: AGN disk reverberation mapping of a supermassive black hole binary candidate

Abstract

We present an intensive multiwavelength monitoring campaign of the quasar PG 1302$-$102 with Swift and the Las Cumbres Observatory network telescopes. At $z\sim0.3$, it tests the limits of the reverberation mapping (RM) technique in probing the accretion disk around a supermassive black hole (SMBH) and extends the parameter space to high masses and high accretion rates. This is also the first time the RM technique has been applied to test disk structures predicted in the SMBH binary model that has been suggested for this source. PG 1302$-$102 was observed at a $\sim$daily cadence for $\sim 9$ months in 14 bands spanning from X-ray to UV and optical wavelengths, and it shows moderate to significant levels of variability correlated between wavelengths. We measure the inter-band time lags which are consistent with a $\tau \propto \lambda^{4/3}$ relation as expected from standard disk reprocessing, albeit with large errors. The disk size implied by the lag spectrum is consistent with the expected disk size for its black hole mass within uncertainties. While the source resembles other reverberation-mapped AGN in many respects, and we do not find evidence supporting the prevalent hypothesis that it hosts an SMBH binary, we demonstrate the feasibility of studying SMBH binaries from this novel angle and suggest possibilities for the LSST Deep Drilling Fields.