Investigating the high-frequency spectral features of SNRs Tycho, W44, and IC443 with the Sardinia Radio Telescope

Abstract

The main characteristics in the radio continuum spectra of Supernova Remnants\n(SNRs) result from simple synchrotron emission. In addition, electron\nacceleration mechanisms can shape the spectra in specific ways, especially at\nhigh radio frequencies. These features are connected to the age and the\npeculiar conditions of the local interstellar medium interacting with the SNR.\nWhereas the bulk radio emission is expected at up to $20-50$ GHz, sensitive\nhigh-resolution images of SNRs above 10 GHz are lacking and are not easily\nachievable, especially in the confused regions of the Galactic Plane. In the\nframework of the early science observations with the Sardinia Radio Telescope\nin February-March 2016, we obtained high-resolution images of SNRs Tycho, W44\nand IC443 that provided accurate integrated flux density measurements at 21.4\nGHz: 8.8 $\\pm$ 0.9 Jy for Tycho, 25 $\\pm$ 3 Jy for W44 and 66 $\\pm$ 7 Jy for\nIC443. We coupled the SRT measurements with radio data available in the\nliterature in order to characterise the integrated and spatially-resolved\nspectra of these SNRs, and to find significant frequency- and region-dependent\nspectral slope variations. For the first time, we provide direct evidence of a\nspectral break in the radio spectral energy distribution of W44 at an\nexponential cutoff frequency of 15 $\\pm$ 2 GHz. This result constrains the\nmaximum energy of the accelerated electrons in the range $6-13$ GeV, in\nagreement with predictions indirectly derived from AGILE and \\textit{Fermi}-LAT\ngamma-ray observations. With regard to IC443, our results confirm the\nnoticeable presence of a bump in the integrated spectrum around $20-70$ GHz\nthat could result from a spinning dust emission mechanism.\n