The interstellar medium in the direction of the Crab Nebula - Reconciling soft X-ray and radio observations

Details The interstellar medium in the direction of the Crab Nebula - Reconciling soft X-ray and radio observations The interstellar medium in the direction of the Crab Nebula - Reconciling soft X-ray and radio observations

Nov 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977a%26a....61..347r&link_type=abstract

Astronomy and Astrophysics, vol. 61, no. 3, Nov. 1977, p. 347-352.

Astronomy and Astrophysics

Astronomy

7

Absorption Cross Sections, Crab Nebula, Interstellar Matter, Radio Astronomy, X Ray Astronomy, Abundance, Cosmic Dust, Radiant Flux Density

Scientific paper

The total soft X-ray photoabsorption cross section of the interstellar medium (ISM) in the direction of the Crab Nebula is computed on the basis of a two-phase model of the ISM. This cross section is used to reanalyze Copernicus data on the X-ray spectrum of the Crab between 0.7 and 1.5 keV. The total hydrogen column density along the line of sight to that nebula is found to be approximately 2.6 by 10 to the 21st power H atoms/sq cm. This result is evaluated in light of the two-phase model of the ISM, and the predictions based on the X-ray data are compared with results of radio and UV observations. A discrepancy between the radio and X-ray measurements of the hydrogen column density is resolved by noting that 21-cm absorption measurements sample only the neutral hydrogen in clouds while X-ray measurements are sensitive to all forms of hydrogen in both cloud and intercloud regions. It is suggested that roughly 50% of the hydrogen in the direction of the Crab Nebula is in clouds and that 85% of this hydrogen is neutral and atomic.

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