Modelling the radio spectra of spatially-resolved Supernova Remnants at high frequencies

The main characteristics of the radio continuum spectra of Supernova Remnants (SNRs) result from simple synchrotron emission. In addition, electron acceleration mechanisms can shape the spectra in specific ways, especially at high radio frequencies. These features are connected to the age and the peculiar conditions of the local ISM interacting with the SNR. Whereas the bulk radio emission is expected at up to 20 − 50 GHz, sensitive high- resolution images of SNRs above ∼ 10 GHz are lacking and are not easily achievable. Indeed, interferometric observations are certainly unbeatable in terms of image resolution, but synthesis imaging becomes difficult in the context of extended sources (10 − 30 arcmin) at high radio frequencies (above a few GHz). On the other hand, sensitive radio continuum observations can be performed with single-dish telescopes, offering in this way a good trade-off between sensitivity and resolution in the frequency range 5 − 50 GHz. The Sardinia Radio Telescope (SRT) represents a great instrument in this context, thanks to its active surface that characterises the primary 64-m-diameter mirror, and which improves spatial resolution at high frequencies. We observed the middle-aged SNRs W44 and IC443 with SRT at 1.55 GHz (L-band), 7.0 GHz (C-band) and 21.4 GHz (K-band). We also observed the young Tycho SNR at 21.4 GHz. These observations provided, for the first time, single-dish high-resolution images of W44 and IC443 at 7 GHz and 21.4 GHz, which demonstrated that a good mapping quality of large structures can be maintained with SRT in the single-dish configuration at up to high-radio frequencies. Our images provided integrated sensitive flux density measurements that we coupled with the radio data available in the literature in order to characterise the integrated and spatially-resolved spectra of these SNRs. We performed spatially-resolved spectral measurements of W44 and IC443 by coupling 1.5 GHz and 7.0 GHz maps. For both W44 and IC443, the obtained spectral index maps reveal a spread in the spectral slope distribution. We coupled the SRT measurements with radio data available in the literature in order to characterise the integrated spectra of these SNRs at up to high-radio frequencies. We provided, for the first time, direct evidence for spectral break in the radio spectral energy distribution of W44 at an exponential cut-off frequency of 15 ± 2 GHz. This result constrains the maximum energy of the accelerated cosmic-ray electrons in the range 6 − 13 GeV, in agreement with predictions indirectly derived from AGILE and Fermi-LAT gamma-ray observations. With regard to IC443, our results confirm the noticeable presence of a bump in the integrated spectrum around 20 − 70 GHz that could result from a spinning dust emission mechanism. In view of the results obtained with SRT in the context of single-dish imaging of extended sources, we decided to observe the very extended and interesting SNR Cygnus Loop at 8.3 GHz with the Medicina radio telescope. From these observations, we obtained the integrated flux density measurement of Cygnus Loop at the highest frequency carried out so far with a single-dish telescope. Although comparable in age, our observations show that W44, IC443 and Cygnus Loop present different spectral features at high radio frequencies. We ascribed this evidence to the environmental differences between these SNRs, which could lead to an enhancement of the secondary electron populations produced by hadronic interactions in regions where the SNR shock collides with dense molecular clouds.