The structure of the Cygnus Loop at 34.5 MHz

Abstract

The large supernova remnant Cygnus Loop was observed at 34.5 MHz with a low-frequency radio telescope at Gauribidanur, India. The structure of the object is presented on a radio map with a resolution of 26 arcmin x 40 arcmin; radio fluxes of different parts of the nebula were measured and used to construct their spectra. The total flux density of the Cygnus Loop at this frequency was 1245 + or - 195 Jy. It is found that the low-frequency spectrum of region C (NGC 6992/5) is not flat below 200 MHz; both the regions B (NGC 6960) and C (NGC 6992/5) exhibit breaks in their spectra around 1000 MHz and 400 MHz, respectively, while the spectrum of region A (NGC 6974) is straight in the entire frequency range. If the spectra of regions B and C are just a reflection of the background radio spectrum of the galaxy, the difference in their break frequencies may be due to their different magnetic field strengths. If, however, the breaks in the spectra are due to synchrotron radiation losses, then the difference in the two regions implies dissimilar ambient interstellar densities.