Microarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions

TL;DR

This study uses VLBI astrometry to precisely measure distances to two binary millisecond pulsars, revealing discrepancies with pulsar timing measurements likely caused by solar wind modeling errors.

Contribution

The paper demonstrates high-precision VLBI distance measurements for two binary MSPs and identifies potential causes for discrepancies with timing-based measurements.

Findings

VLBI measured distances: 700+14-10 pc and 613+16-14 pc

Discrepancies with pulsar timing measurements up to 5 sigma

Imperfect solar wind modeling likely causes timing errors

Abstract

Model-independent distance constraints to binary millisecond pulsars (MSPs) are of great value to both the timing observations of the radio pulsars, and multiwavelength observations of their companion stars. Very Long Baseline Interferometry (VLBI) astrometry can be employed to provide these model-independent distances with very high precision via the detection of annual geometric parallax. Using the Very Long Baseline Array, we have observed two binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-year period and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pc respectively. We use the well-calibrated distance in conjunction with revised analysis of optical photometry to tightly constrain the nature of their massive (M ~ 0.85 Msun) white dwarf companions. Finally, we show that several measurements of their parallax and proper motion of PSR J1022+1001 and PSR J2145-0750 obtained by pulsar timing array projects are incorrect, differing from the more precise VLBI values by up to 5 sigma. We investigate possible causes for the discrepancy, and find that imperfect modeling of the solar wind is a likely candidate for the timing model errors given the low ecliptic latitude of these two pulsars.