We have used the NRAO Very Long Baseline Array ( VLBA ) to image OH ( 1720 MHz ) masers in the supernova remnants W28 and W44 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 can not 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 II regions .
At two locations in W28 we observe the classical S-shapes in the Stokes V profiles caused by Zeeman splitting and use it to infer magnetic fields of order 2 milliGauss .