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Title:
Spatially Resolved Thermal Continuum Absorption against Supernova Remnant W49B
Authors:
Lacey, C. K.; Lazio, T. Joseph W.; Kassim, Namir E.; Duric, N.; Briggs, D. S.; Dyer, K. K.
Affiliation:
AA(National Research Council and Naval Research Laboratory, Code 7213, Washington, DC 20375-5351 ), AB(Naval Research Laboratory, Code 7213, Washington, DC 20375-5351 , ), AC(Naval Research Laboratory, Code 7213, Washington, DC 20375-5351 , ), AD(Department of Physics and Astronomy, University of New Mexico, 800 Yale Boulevard Northeast, Albuquerque, NM 87131), AE(National Research Council and Naval Research Laboratory, Code 7210, Washington, DC 20375-5351), AF(North Carolina State University, Raleigh, NC 27695; )
Publication:
The Astrophysical Journal, Volume 559, Issue 2, pp. 954-962. (ApJ Homepage)
Publication Date:
10/2001
Origin:
UCP
ApJ Keywords:
ISM: individual (W49B), ISM: Structure, Radio Lines: ISM, Scattering, ISM: Supernova Remnants
DOI:
10.1086/322372
Bibliographic Code:
2001ApJ...559..954L

Abstract

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 (~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.
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