DA 530: A supernova remnant in a stellar wind bubble

The high-latitude supernova remnant (SNR) DA 530 (G93.3 + 6.9), apparently a typical shell remnant, has highly polarized radio continuum emission and a very uniform circumferential magnetic held. We present new radio continuu m (408 and 1420 MHz) and H I line observations, made with the Dominon Radio Astrophysical Observatory Synthesis Telescope, and we have made the first detection of X-ray emission from the SNR, using the ROSAT Position-Sensitiv e Proportional Counter. The SNR lies within a shell of H I, possibly create d by an earlier stellar wind, whose kinematic distance is nominally 2.5 kpc but whose actual distance may be larger. The X-ray emission is extremely f aint. A Raymond-Smith ionization-equilibrium model fits the data and sugges ts a very low density, similar to 0.05 cm(-3), consistent with the occurren ce of the supernova in a stellar wind cavity, but this model yields an expl osion energy 100 times lower than the accepted value. A nonequilibrium shoc k model, incorporating a range of ionization timescales, is able to give mo re realistic physical parameters for the supernova remnant. On the balance of the evidence, we place DA 530 at a distance of 3.5 kpc, the largest dist ance permitted by the H I observations, where it lies 420 pc above the Gala ctic plane. The explosion, probably a Type Ia supernova, in a low-density c avity has resulted in weak X-ray emission and slow evolution. The explosion energy was 3.9 x 10(50) ergs and the age is similar to 5000 years. The rem nant, having swept up 3.9 M. in an ambient density of similar to 0.01 cm(-3 ), is only now in the adiabatic phase, and this explains the absence of det ected optical emission. Despite the low ambient density the efficiency of g eneration of synchrotron radio emission is similar to 0.4%, higher than in some historical SNRs. The ratio of radio to X-ray flux is about 100 times t hat for the remnant of SN 1006, which has comparable radio continuum proper ties. The very uniform magnetic field is not explained. DA 530 joins a smal l group of remnants at high Galactic latitude with unusual features, perhap s resulting from low ambient densities. Inhomogeneous nonequilibrium ioniza tion models may be required for the interpretation of the X-ray emission fr om many other older SNRs.