The Astrophysical Journal Supplement Series, 71:799-815, 1989

RADIO SPECTRUM STUDIES OF 32 FIRST-QUADRANT GALACTIC SUPERNOVA REMNANTS

Namir Kassim

We present a comprehensive list of integrated radio continuum flux density measurements for 32 first-quadrant Galactic supernova remnants (SNRs). The list includes all available measurements from the literature, as well as some previously unpublished low-frequency measurements from the Clark Lake Galactic plane survey. An measurements have been placed on the same absolute flux density scale, and those which are deemed poor or otherwise inappropriate for the purposes of constructing continuum spectra have been noted. This compilation has been used in a separate paper to construct accurate spectra and to constrain the distribution and physical properties of ionized gas in the interstellar medium.

Subject headings : nebulae: supernova remnants - radio sources: general - radio sources: spectra

I. INTRODUCTION

In order to use an individual SNR to measure or set limits on the opacity of any intervening ionized material, its entire radio spectrum must be accurately determined. Higher frequency (nu >= 300 MHz) measurements are necessary to determine the intrinsic spectral index a which is then used, in conjunction with lower frequency (usually < 100 MHz) measurements, to determine the free-free optical depth Tau. Unfortunately, the information necessary to construct the entire spectrum of individual SNRs, specifically the measured values of the integrated flux densities, are often widely scattered throughout the literature and are thus not readily available. Moreover, flux densities for individual sources obtained from observations made by different authors using different instruments are rarely based on the same absolute flux density scale. Finally, many published flux density measurements are not appropriate for determining continuum spectra, usually for a variety of instrumental reasons.

In this paper, we have gathered all available integrated flux densities for 32 first-quadrant Galactic SNRs, including several previously unpublished low-frequency measurements made with the Clark Lake TPT telescope (Erickson, Mahoney, and Erb 1982; Kassim 1989, and references therein). All flux densities have been placed on the same absolute flux density scale, and poor or inappropriate measurements have been noted. The data presented here are used in a separate paper (Kassim 1989) to construct accurate spectra for these 32 sources, which in turn can be used to constrain the distribution and properties of low-density ionized gas in the ISM. Parameters from the derived spectra are also included here.

a) Data

For each SNR we have also obtained the parameters a, Tau408, and S408 from a least-squares fit to the equation:

S_nu = [ S408 (nu/408)^a] exp[-Tau408(nu/408)^-2.1 ], (1)
where S_v is the integrated flux density in Janskys at frequency nu in MHz and S408 and Tau408 are the flux density and optical depth at the reference frequency of 408 MHz, respectively.

The intrinsic spectral index of the SNR (a) is assumed to be constant throughout the radio range. In the headers of Table 1 we have scaled the derived optical depths to 30.9 MHz (Tau30.9) by multiplication of Tau408 the appropriate conversion factor of (30.9/408)^-2.1 (see Kassim 1989, and references therein).

As expected, Tau30.9 is most sensitive to the accuracy of the lowest frequency flux density measurements, i.e., those from Clark Lake, so that the errors for Tau have been obtained from the ~20% error estimate appropriate to the Clark Lake data (Kassim 1987, 1988; see also Kassim 1989). While Tau is also dependent on the spectral index of the SNR and therefore on the higher frequency measurements, in most cases numerous higher frequency points are available to determine the optically thin spectral index, making Tau relatively insensitive to the error of any individual measurement.

Table 1 lists all available integrated flux density measurements. Columns (1), (2), and (3) list, respectively, the observing frequency in MHz, the corrected (see below) integrated flux density (Sc) in Janskys (underlined values are not used; see below), and an error estimate (also in Janskys) if one is available. Columns (4) and (5) list the instrument and reference for each observation, and column (6) gives the correction factor which was used to bring the published flux density to the absolute scale of Baars et al. (1977). Most of these correction factors were obtained from Kuhr et al. (1981). The remainder was derived by comparing the assumed flux densities of the original calibrators with their values determined by Baars et al. (1977). The notation "NA" in column (7) means that an estimate of the correction factor is not available, usually because the original reference does not list the assumed flux densities of the primary calibrators. (In some cases note that only a secondary reference or instrument description is listed in col. [4].)

The flux densities listed in Table 1 have also been edited to remove poor or inappropriate measurements. The principal rejection criteria were the following:

1. The observations have been superseded by more recent, higher quality observations at a comparable frequency, or the observations are in strong disagreement with a number of other measurements made at nearby frequencies.
2. The observations were made with an instrument having insufficient resolution to avoid confusion from nearby sources. Confusion by thermal sources often leads to an overestimate of the flux density at high frequencies.
3. The observations were made by an interferometer with insufficient short spacings to measure the full integrated flux density of a large remnant. This undersampling leads to an underestimate of the flux density.

A discussion of the analysis and interpretation of these data and plots of the points and fitted spectra for the SNRs within the radio regime are given in Kassim (1989). Results of that analysis are inconsistent with low-frequency absorption of SNRs by a widely distributed, homogeneous ionized component of the ISM. Instead they suggest that low-frequency turnovers in SNR spectra are due to absorption by localized regions of low-density ionized gas probably associated with normal H II regions.

The author would like to thank William C. Erickson and Kurt W. Weiler for many useful discussions and comments. The Clark Lake Radio Observatory was supported by the National Science Foundation under grant AST-8416179. Part of this work was taken from a thesis submitted in partial fulfillment of the requirements for the Ph.D. degree in the department of Physics and Astronomy at the University of Maryland.

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