Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spatially-resolved Thermal Continuum Absorption against the Supernova
  Remnant W49B   We present sub-arcminute 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 a 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 HII regions surrounded either partially or
fully by low-density HII gas. We identify this low-density gas as an extended
HII region envelope (EHE), whose statistical properties were inferred from low
resolution meter- and centimeter-wavelength recombination line observations.
Comparison of our radio images with HI and H_2CO observations show that the
intervening thermal gas is likely associated with neutral and molecular
material as well.

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The categories of this paper are: mtrl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-lat
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The categories of this paper are: hep-ex
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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The categories of this paper are: econ
1
The categories of this paper are: cs
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The categories of this paper are: acc-phys
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The categories of this paper are: eess
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The categories of this paper are: gr-qc
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: chao-dyn
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The categories of this paper are: nlin