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M.J. Claussen, W.M. Goss, D.A. Frail, K. Desai (NRAO)
We have used the NRAO Very Long Baseline Array (VLBA) to image OH(1720 MHz) masers in the supernova remnants W\thinspace{28} and W\thinspace{44} at a resolution of 40 mas. We also used MERLIN to observe the same OH(1720 MHz) masers in W44 at a resolution of 290 \times 165 mas. All the masers are resolved by these VLBA and MERLIN observations. The measured sizes range from 50 to 180 mas and yield brightness temperature estimates from 0.3---20\times{10}8 K. We investigate whether these measured angular sizes are intrinsic and hence originate as a result of the physical conditions in the supernova remnant shock, or whether they are scatter broadened sizes produced by the turbulent ionized gas along the line of sight. While the current data on the temporal and angular broadening of pulsars, masers and extragalactic sources toward W44 and W28 can be understood in terms of scattering, we cannot rule out that these large sizes are intrinsic. Recent theoretical modeling by Lockett et~al. suggests that the physical parameters in the shocked region are indicative of densities and OH abundances which lead to estimates of sizes as large as what we measure. If the sizes and structure are intrinsic, then the OH(1720 MHz) masers may be more like the OH(1612 MHz) masers in circumstellar shells than OH masers associated with H\hskip 2trueptII regions. At two locations in W\thinspace{28} we observe the classical S-shape in the Stokes V profiles caused by Zeeman splitting and use it to infer magnetic fields of order 2 milliGauss.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. MERLIN is a UK national facility operated by the University of Manchester on behalf of PPARC.
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