improving pulsar timing array sensitivity using vlbi astrometry

Pulsars are neutron stars that spin very quickly, with the fastest of them spinning 100s of times in a second. They are very stable, cosmic clocks. Pulsar Timing Arrays continuously monitor a population of galactic pulsars using radio telescopes. Energetic events (like the mergers of 2 supermassive black holes) happening in the Universe result in deviations from pulse arrival times - and these are signs of exciting astrophysics!

There are many Pulsar Timing Array consortia around the world, including the Indian Pulsar Timing Array Experiment (InPTA), of which I have been a member since 2023. These experiments montior radio pulsars, and search for signals pointing to a low frequency nanoHz Gravitational Wave background. In June 2023, InPTA combined its data with the EPTA dataset and detected strong HD correlation, which provides strong evidence of stochastic GW Background. Other PTA experiments reported the same.

It is then crucial to keep improving PTA sensitivity to possible GW events so that we may one day move to a detection. The sensitivity of pulsar timing array (PTA) experiments to detect gravitational waves at and around the 1 yr^-1 frequency is limited by the fact that the pulsars’ astrometric parameters are a priori poorly constrained. This sensitivity can be improved by combining PTA datasets with independent astrometric measurements, such as those obtained using very-long-baseline interferometry (VLBI).

Unfortunately, VLBI measurements are made in a coordinate system that differs from that used for pulsar timing by an a priori unknown small rotation and cannot be used directly as prior distributions in PTA Bayesian inference. We present a new method for rigorously incorporating VLBI measurements in PTA gravitational wave analyses, where this coordinate rotation is explicitly handled, and show that this leads to marked improvement for the PTA GW sensitvity by using a simulated dataset of pulsars.