Spatially Resolved Thermal Continuum Absorption Against Supernova Remnant W49B
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
Pagination or Media Count:
We present subarcminute resolution imaging of the Galactic supernova remnant W49B at 74 MHz 25 and 327 MHz 6, the former being the lowest frequency at which the source has been resolved. While the 327 MHz image shows a shell-like morphology similar to that seen at higher frequencies, the 74 MHz image is considerably different, with the southwest region of the remnant almost completely attenuated. The implied 74 MHz optical depth approximately 1.6 is much higher than the intrinsic absorption levels seen inside two other relatively young remnants, Cas A and the Crab Nebula, nor are natural variations in the relativistic electron energy spectra expected at such levels. The geometry of the absorption is also inconsistent with intrinsic absorption. We attribute the absorption to extrinsic free-free absorption by an intervening cloud of thermal electrons. Its presence has already been inferred from the low-frequency turnover in the integrated continuum spectrum and from the detection of radio recombination lines toward the remnant. Our observations confirm the basic conclusions of those measurements, and our observations have resolved the absorber into a complex of classical H II regions surrounded either partially or fully by low-density H II gas. We identify this low-density gas as an extended H II region envelope EHE, whose statistical properties were inferred from low-resolution meter- and centimeter-wavelength recombination line observations. Comparison of our radio images with H I and H2CO observations shows that the intervening thermal gas is likely associated with neutral and molecular material as well. This EHE may be responsible for the enhanced radio-wave scattering seen in the general direction of the W49 complex.