A Possible Detection of Gravitational Waves in the Correlation Between Quasar Proper Motions and Pulsar Timing

Telescopes can detect gravitational waves by monitoring astrophysical clocks or measuring the astrometric deflections of light sources. "Earth frame" gravitational waves will create pulsar timing drifts and apparent proper motions of stars, galaxies, and quasars. The line-of-sight and transverse motions induced by gravitational waves have a well-defined correlation as a function of their angular separation, an analog of the Hellings-Downs curve that is seen in the pulsar timing-based detection of the stochastic gravitational wave background. Here, we describe a first attempt to combine pulsar timing residuals and quasar proper motions to detect nHz gravitational waves. We find an angular correlation pattern that agrees well with predictions and has an intensity and characteristic strain that is consistent with the pulsar timing results. There are, however, numerous systematic effects that could produce a false detection; we examine these and do not find an obvious source of a spurious signal. There is also a null-valued control measurement that does not show the Hellings-Downs correlation. We will discuss the process and results of this investigation and suggest next steps to confirm or refute the possible detection of the stochastic gravitational wave background.