# Techniques for Accurate Parallax Measurements for 6.7-GHz Methanol Masers

## TL;DR

This paper introduces an analysis method to improve parallax measurements of 6.7-GHz methanol masers by mitigating ionospheric effects, enhancing the accuracy of Galactic structure mapping.

## Contribution

The paper presents a novel technique using an artificial quasar to reduce ionospheric errors in methanol maser parallax measurements.

## Key findings

- Significant reduction in parallax gradient effects.
- Improved accuracy in maser distance measurements.
- Enhanced mapping of the Milky Way's spiral structure.

## Abstract

The BeSSeL Survey is mapping the spiral structure of the Milky Way by measuring trigonometric parallaxes of hundreds of maser sources associated with high-mass star formation. While parallax techniques for water masers at high frequency (22 GHz) have been well documented, recent observations of methanol masers at lower frequency (6.7 GHz) have revealed astrometric issues associated with signal propagation through the ionosphere that could significantly limit parallax accuracy. These problems displayed as a "parallax gradient" on the sky when measured against different background quasars. We present an analysis method in which we generate position data relative to an "artificial quasar" at the target maser position at each epoch. Fitting parallax to these data can significantly mitigate the problems and improve parallax accuracy.