OH Maser sources in W49N: probing differential anisotropic scattering with Zeeman pairs

TL;DR

This study uses high-resolution VLBA observations of OH masers in W49N to analyze anisotropic scattering, revealing less scattering than previously thought and polarization-dependent differences that inform about the galactic magnetic field and intervening medium.

Contribution

First detailed high-resolution imaging of OH masers in W49N revealing anisotropic scattering and polarization-dependent effects, challenging earlier scattering estimates.

Findings

Anisotropic scatter broadening with axial ratios 1.5 to 3.

Average angular sizes are 2.5 times smaller than previous reports.

Significant polarization-dependent differences in scatter broadening for Zeeman pairs.

Abstract

Our analysis of a VLBA 12-hour synthesis observations of the OH masers in W49N has provided detailed high angular-resolution images of the maser sources, at 1612, 1665 and 1667 MHz. The images, of several dozens of spots, reveal anisotropic scatter broadening; with typical sizes of a few tens of milli-arc-seconds and axial ratios between 1.5 to 3. The image position angles oriented perpendicular to the galactic plane are interpreted in terms of elongation of electron-density irregularities parallel to the galactic plane, due to a similarly aligned local magnetic field. However, we find the apparent angular sizes on the average a factor of 2.5 less than those reported by Desai et al., indicating significantly less scattering than inferred earlier. The average position angle of the scattered broadened images is also seen to deviate significantly (by about 10 degrees) from that implied by the magnetic field in the Galactic plane. More intriguingly, for a few Zeeman pairs in our set, we find significant differences in the scatter broadened images for the two hands of polarization, even when apparent velocity separation is less than 0.1 km/s. Here we present the details of our observations and analysis, and discuss the interesting implications of our results for the intervening anisotropic magneto-ionic medium, as well as a comparison with the expectations based on earlier work.