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Astron. Astrophys. 324, 641-650 (1997)

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Multi-frequency radio spectral studies of the supernova remnant G78.2+2.1

X. Zhang 1, Y. Zheng 1, T.L. Landecker 2 and L.A. Higgs 2

1 Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China
2 Dominion Radio Astrophysical Observatory, National Research Council, P.O. Box 248, Penticton, British Columbia V2A 6K3, Canada

Received 10 January 1997 / Accepted 14 February 1997

Abstract

A new map of the SNR G78.2+2.1 at 232 MHz is presented. This map is used with other images at 408, 1420, 2695 and 4800 MHz to investigate the total spectral index of the SNR and the variation of spectral index across it. The integrated spectral index is [FORMULA] (where [FORMULA]). Maps of the spectral-index variation across the source are derived using a method which calculates a differential spectral index. This method is independent of offset errors and large-scale background variations. Systematic spectral-index variations of [FORMULA] 0.15 are found, consistently in the many independent pairs of data sets. Regions of higher spectral index (steeper spectra) appear around the periphery of the remnant and in its central area. Regions of lower spectral index (flatter spectra) are also found within the remnant. The latter may result from a change in the nature of the non-thermal emission or may represent "contamination" from underlying patches of thermal emission. There is no clear correlation of spectral index with intensity across the SNR. Theoretical reasons for spectral-index variations have been explored but no clear interpretation of the apparent spectral steepening near the shock front has been found nor has an explanation of the steep-spectrum central region.

Key words: ISM: supernova remnants – ISM: individual objects: G78.2+2.1

Send offprint requests to: L.A. Higgs

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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