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.
