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An archival ROSAT XRT/HRI image of the Cassiopeia~A supernova remnant was corrected for absorption, on 30\arcsec \ (0.5 pc) scales, using existing OH and H~I absorption data (Bieging \& Crutcher 1986, Goss unpublished data ), and theoretical effective x-ray cross-sections for the ISM. A much stronger correlation was discovered than had been previously seen between the x-ray and radio emission. We observe $\frac{\log(radio)}{\log(x-ray)}$ values between 0.26 and 0.37. The implications of these data for physical parameters in the relativistic and thermal plasmas of the supernova remnant will be discussed.
Assuming the relation above, the $\lambda20~cm$ radio emission, which is free from absorption effects, was then used as a predictor of the actual x-ray emission. By comparing the predicted values to the uncorrected x-ray image, an empirical x-ray optical depth was derived. A good linear correlation exists between this empirical x-ray optical depth and the column density of hydrogen as calculated using the OH and H~I absorption data. The effective cross-section of the interstellar medium was then calculated and found to be consistent with the theoretical models.
This work is supported in part by NSF grant AST 9100486 to the University of Minnesota.
