OH Zeeman Magnetic Field Detections Toward Five Supernova Remnants Using the VLA

Abstract

We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to measure their magnetic field strengths using the Zeeman effect. We detected all 12 of the bright ($S_ν > 200$ mJy) OH (1720 MHz) masers previously detected by Frail et al. (1996) and Green et al. (1997) and measured significant magnetic fields (i.e. $ > 3σ$) in ten of them. Assuming that the ``thermal'' Zeeman equation can be used to estimate $\mid\vec{B}\mid$ for OH masers, our estimated fields range from 0.2 to 2 mG. These magnetic field strengths are consistent with the hypothesis that ambient molecular cloud magnetic fields are compressed via the SNR shock to the observed values. Magnetic fields of this magnitude exert a considerable influence on the properties of the cloud with the magnetic pressures ($10^{-7} - 10^{-9}$ erg cm$^{-3}$) exceeding the pressure in the ISM or even the thermal pressure of the hot gas interior to the remnant. This study brings the number of galactic SNRs with OH (1720 MHz) Zeeman detections to ten.