First astrometric constraints on parity-violation in the gravitational wave background

Astrometry, the precise measurement of stellar positions and velocities, offers a promising approach to probing the low-frequency stochastic gravitational wave background (SGWB). Notably, astrometric vector sky maps are sensitive to parity-violating SGWB signals, which cannot be distinguished using pulsar timing array observations in an isotropic SGWB. We present the first astrometric constraints on parity-violating SGWB using quasar catalogs from Gaia DR3 and VLBA data. By analyzing the $EB$ correlation in the two-point correlation function of the proper motions of the quasars, we find 2$\sigma$ constraints on the parity-violating SGWB amplitude $h_{70}^2{\mathrm{\Omega }}_V=-0.020\pm 0.025$ from Gaia DR3 and $h_{70}^2{\mathrm{\Omega }}_V=-0.004\pm 0.010$ from VLBA. These constraints are valid in the frequency range $4.2\times {10}^{-18}\mathrm{H}\mathrm{z}<f<1.1\times {10}^{-8}\mathrm{H}\mathrm{z}$. Although not currently a tight constraint on theoretical models, this first attempt lays the groundwork for future investigations using more precise astrometric data.