Interplanetary scintillation-informed heliospheric modelling for the MeerKAT Pulsar Timing Array 4.5 yr dataset

Abstract

Heliospheric density variations impart delays on pulse times of arrivals from millisecond pulsars. Improper modelling of these variations may affect gravitational wave detection and characterisation by pulsar timing arrays (PTAs). Currently, PTAs typically employ a time-varying, spherically symmetric heliosphere model, which does not capture the full spatial and temporal complexity of the heliosphere. Instead, we investigate whether a three-dimensional, time-dependent model of the inner heliosphere from interplanetary scintillation (IPS) measurements - the IPS-UCSD model - can be employed to mitigate the solar wind in PTA analyses. We applied the IPS-UCSD model to the MeerKAT PTA 4.5-year dataset to assess whether it could correct for heliospheric density variations, and the impact on GW sensitivity compared to a spherically-symmetric model. We find that the model does not accurately correct for heliosphere-induced timing distortions, leading to bias in recovered GW parameters. Using simulations, we show that the spherically symmetric heliosphere model also fails to fully capture heliospheric density variations like those in the IPS-UCSD model. However, if interstellar dispersion measure (DM) variations are also modelled, then the heliospheric model errors are partially absorbed by DM variations, reducing contamination of the GW signal. Therefore we find that a time-varying spherically symmetric model is sufficient to mitigate the effect of heliospheric time delays on recovered GW results at typical PTA radio frequencies, provided other signal components are also modelled. We propose that the most precisely timed pulsars may be used to improve data-driven heliospheric density models in the future.