On the angular localization of gravitational-wave signals by pulsar timing arrays

Abstract

We provide a complete study of gravitational-wave signal localization using pulsar timing arrays. We derive analytical expressions for the Cram\'er-Rao sky localization precision that delineate the impact of the angular proximity, $\xi$, between the pulsar and the gravitational wave source, and the precision, $\sigma_L$, with which pulsar distances are known. Interference between the Earth and pulsar terms creates rapid angular oscillations for sky-coordinate Fisher matrix elements that aid localization, which is complemented by more broadly varying antenna response gradient information. The relative importance of these factors depends on whether pulsar distances are known precisely [i.e., $\sigma_L\leq\lambda_\mathrm{GW}/(1-\cos\xi)$] or imprecisely. If pulsar distances are known precisely, tightening this distance precision improves signal localization according to $\Delta\Omega_\mathrm{sky}\propto(\sigma_L/L)^2$ until the Earth-pulsar system reaches its diffraction limit, $\Delta\Omega_\mathrm{sky}\propto(\lambda_\mathrm{GW}/L)^2$. If pulsar distances are not known well, localization precision is degraded, but more pulsars in close proximity to the source is the best means of improving. With $\alpha$ indexing pulsars, this scales as $\Delta\Omega_\mathrm{sky}~\propto~(\sum_\alpha \mathrm{SNR}_\alpha^2/\xi_\alpha^2)^{-1}$ in the small-angle limit of the unmarginalized Fisher matrix, and we derive the analytic generalization to any angle between a pulsar and the source. Finally, we study a scenario where pulsar-term phases are treated as nuisance variables that are unconnected to binary or PTA properties. This phase-decoupled scenario, which is how all PTA continuous wave searches are currently conducted, delivers localization performance similar to the antenna-response--driven case, and does not exhibit significant improvement as pulsar distance precisions are tightened.