A Mote in Andromeda's Disk: a Misidentified Periodic AGN Behind M31

We identify an object previously thought to be a star in the disk of M31, J0045+41, as a background $z\approx0.215$ AGN seen through a low-absorption region of M31. We present moderate resolution spectroscopy of J0045+41 obtained using GMOS at Gemini-North. The spectrum contains features attributable to the host galaxy. We model the spectrum to estimate the AGN contribution, from which we estimate the luminosity and virial mass of the central engine. Residuals to our fit reveal a blue-shifted component to the broad H$\alpha$ and H$\beta$ at a relative velocity of $\sim4800$ km s$^{-1}$. We also detect \ion{Na}{1} absorption in the Milky Way restframe. We search for evidence of periodicity using $g$-band photometry from the Palomar Transient Factory and find evidence for multiple periodicities ranging from $\sim80-350$ days. Two of the detected periods are in a 1:4 ratio, which is identical to the predictions of hydrodynamical simulations of binary supermassive black hole systems. If these signals arise due to such a system, J0045+41 is well within the gravitational wave regime. We calculate the time until inspiral due to gravitational radiation, assuming reasonable values of the mass ratio of the two black holes. We discuss the implications of our findings and forthcoming work to identify other such interlopers in the light of upcoming photometric surveys such as the Zwicky Transient Facility (ZTF) or the Large Synoptic Survey Telescope (LSST) projects.