Thanks to its large size, its location well out of the Galactic plane, and its high brightness, the middle-age SNR Cygnus Loop is well suited for observations across the entire electromagnetic spectrum. In the radio band, this SNR presents a very characteristic morphology with a large northern circular shell and a bubble-like shell located in the southern part. The radio studies showed substantial differences between these two regions, which suggested that the Cygnus Loop consists of two interacting supernova remnants. Indeed, the northern part of the remnant would seem governed by the compression of the magnetic field due to the SN explosion shock, while the southern part results most likely from shock acceleration that is thought to act in young remnants. Furthermore, a possible spectral steepening above 2.7 GHz is still debated.Despite this scientific interest, sensitive flux density measurements are missing at higher frequencies (above ∼5 GHz), due to the technical difficulties in performing radio continuum observations of a so large sources. These arguments made Cygnus Loop a challenging and very interesting target to observe with the Italian radio telescopes. With this aim, we observed Cygnus Loop with the Medicina radio telescope at 8.3 GHz, obtaining the highest-frequency map so far with a single-dish radio telescope. In this image we detected the two prominent shells and the central filament, which constitute the northern remnant, as well as the southern remnant. Our measurement rules out any global spectral steepening up to 8.3 GHz.At the light of these results, we included Cygnus Loop in our 2018 observing project with the Sardinia Radio Telescope (SRT). The project is focused on the high-frequency and polarimetric comparative investigations on a wider sample of middle-aged and young SNRs in order to perform a precise modelling of the region-dependent continuum spectral indices and to disentangle magnetic enhancement processes from spectral variation contributions. Spectro-polarimetric observations have been performed with SRT at 6.9 GHz and in the range 18.6-24.5 GHz on two interesting regions of Cygnus Loop: the northern-bright filament and the southern remnant. These observations are crucial to study region-dependent variations on spectral indices and polarisation properties above ∼ 5 GHz, which are attributable to different acceleration mechanisms acting in these regions.
Supernova Remnants: An Odyssey in Space after Stellar Death II
- Pub Date:
- June 2019