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Astron. Astrophys. 324, 1152-1164

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The supernova remnant CTA1 (G 119.5+10.3): a study
of the breakout phenomenon

S. Pineault1 and T.L. Landecker2, and C.M. Swerdlyk2, and W. Reich3

1Département de physique et Observatoire du Mont Mégantic, Université Laval, G1K 7P4 Ste-Foy (Québec), Canada
2Dominion Radio Astrophysical Observatory, National Research Council of Canada, P.O. Box 248, Penticton, B.C., Canada
3Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn 1, Germany

Received 5 November 1996 / Accepted 21 February 1997


The supernova remnant (SNR) CTA1 has been observed with the Dominion Radio Astrophysical Observatory Synthesis Telescope, at 1420 MHz and 408 MHz, and with the Effelsberg 100 m telescope at 1420 MHz. Angular resolution of the final maps is FORMULA and FORMULA at 1420 and 408 MHz respectively. New HIRES infrared observations with a resolution of about FORMULA are also presented. Using those new observations, together with a new flux density measured from the 22.25 MHz DRAO array and previously published values for the integrated flux density, we deduce an integrated spectral index of FORMULA (where FORMULA ). The new high-resolution radio maps confirm that, in addition to the bright radio arcs visible to the south and east, the SNR has a very substantial extension to the northwest, interpreted as the breakout of the SNR blast wave into a medium of lower density. Spatial variations are found in the spectral index distribution over the SNR, the diffuse emission to the northwest being generally of a steeper spectral index than emission associated with the brighter regions. Although this can be explained within the context of diffusive shock acceleration theory by variations in the Mach number of the SNR shock, it is not clear whether sufficiently large changes can occur following breakout. Faint emission is also present past the southern limb-brightened radio filaments. This could be the result of projection effects or due to electrons diffusing upstream ahead of the shock front with a mean free path of order 0.02 pc. A break in the southeastern part of the radio shell is best explained by a density enhancement or cloud which has caused a drastic reduction in shock velocity possibly resulting in a decrease in the acceleration efficiency.

Key words: acceleration of particles - ISM: cosmic rays - ISM: individual objects: CTA1 - ISM: supernova remnants - radio continuum: interstellar

Send offprint requests to: S. Pineault

© European Southern Observatory (ESO) 1997