Santiago Jaraba

Astrometric surveys can be used to constrain the stochastic gravitational wave background (SGWB) at very low frequencies. We use proper motion data provided by Gaia DR3 to fit a generic dipole+quadrupole field. We analyse several quasar-based data sets and discuss their purity and idoneity to set constraints on gravitational waves. For the cleanest data set, we derive an upper bound on the (frequency-integrated) energy density of the SGWB $h_{70}^2\Omega _{\rm GW}\lesssim 0.087$ for 4.2 × 10−18 Hz ≲ f ≲ 1.1 × 10−8 Hz. We also reanalyse previous VLBI-based data to set the constraint $h_{70}^2\Omega _{\rm GW}\lesssim 0.024$ for 5.8 × 10−18 Hz ≲ f ≲ 1.4 × 10−9 Hz under the same formalism, standing as the best astrometric constraint on GWs. Based on our results, we discuss the potential of future Gaia data releases to impose tighter constraints.