3. MM-wave continuum radiation from the Cassiopeia A SNR
Maps of the Cas A continuum are shown in Fig. 1. The fluxes we measured were Jy at 86 GHz on 15 December 1997 and Jy at 140 GHz on 5 February 1998, or Jy and Jy reduced to the standard epoch of 1965 (Baars et al., 1977). For a spectral index taken between two frequencies and and defined as , . When the fractional rms errors in the fluxes are and , it follows that the rms error of the spectral index is . For our data, between 86 and 140 GHz.
As shown in Fig. 2 the fluxes we measured at mm-wavelengths agree quite well with the extrapolation of the power-law slope found between 1 and 23 GHz in the classic work of Baars et al. (1977). Our fitted slope is in agreement with their value . This may be somewhat surprising given the strong argument that shock acceleration should create some curvature in the spectrum. The curvature found by Reynolds & Ellison (1992) through comparision of measurements above and below 1 GHz seems not to play a strong role in determining the microwave spectrum above 1 GHz.
Two earlier maps of the 3mm radiation can be found in the literature. Dickel (1978) found Jy at 90 GHz, corrected to the 1997 December epoch of our observations, while Kenney & Dent (1985) observed Jy at 86 GHz, similarly corrected. The latter work thus suggested a significant 3mm excess and a flattening of the apparent spectral index of the nebula between 10 and 90 GHz. They attributed the excess emission to cool dust. We obviously do not confirm this result and ascribe the difference either to faulty calibration or uneven sampling (which was done at 28 positions, many of which lay on strong portions of the radiocontinuum) in the earlier work.
The constancy of the spectral index with frequency suggests that the nebula must be rather uniform when averaged over large beamwidths such as those that we have used (Anderson & Rudnick, 1996). Indeed, we separated the nebula into its stronger and weaker halves by integrating over regions on either side of a line through the center in position angle 50o but the same proportion of the flux resides on either side of the line at both 86 and 140 GHz.
From the contour levels in Fig. 1 it is seen that the brightest portions of the nebula at 86 GHz are only 0.3 K in our 72" beam or 0.11 K at 140 GHz and 44" resolution. This is far too small to provide a background for measurement of the abundances of interstellar molecules via their absorption spectra.
© European Southern Observatory (ESO) 1999
Online publication: June 18, 1999