The Benefits of VLBI Astrometry to Pulsar Timing Array Searches for Gravitational Radiation

TL;DR

This paper demonstrates that VLBI astrometry significantly improves pulsar timing precision and gravitational wave detection by providing independent, high-accuracy position measurements, especially for shorter data sets affected by red noise.

Contribution

It introduces a method to integrate VLBI astrometry with pulsar timing, reducing bias from red noise and enhancing gravitational wave signal detection capabilities.

Findings

VLBI provides sub-milliarcsecond pulsar positions independent of timing noise.

Incorporating VLBI improves gravitational wave signal detection in pulsar timing.

Timing model fitting techniques are effective over multi-year datasets but less so for shorter observations.

Abstract

Precision astrometry is an integral component of successful pulsar timing campaigns. Astrometric parameters are commonly derived by fitting them as parameters of a timing model to a series of pulse times of arrival (TOAs). TOAs measured to microsecond precision over several-year spans can yield position measurements with sub-milliarcsecond precision. However, timing-based astrometry can become biased if a pulsar displays any red spin noise, which can be compared to the red noise signal produced by the stochastic gravitational wave background. We investigate how noise of different spectral types is absorbed by timing models, leading to significant estimation errors in the astrometric parameters. We find that commonly used techniques for fitting timing models in the presence of red noise (Cholesky whitening) prevent the absorption of noise into the timing model remarkably well if the time baseline of observations exceeds several years, but are inadequate for dealing with shorter pulsar data sets. Independent of timing, pulsar-optimized very long baseline interferometry (VLBI) is capable of providing position estimates precise to the sub-milliarcsecond levels needed for high-precision timing. We compute a necessary transformation between the International Celestial Reference Frame and pulsar timing frames and quantitatively discuss how the transformation will improve in coming years. We find that incorporating VLBI astrometry into the timing models of pulsars for which only a couple of years of timing data exist will lead to more realistic assessments of red spin noise and could enhance the amplitude of gravitational wave signatures in post-fit timing residuals by factors of 20 or more.