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 (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 interstellar medium 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, especially in the confused regions of the Galactic Plane. In the framework of the early science observations with the Sardinia Radio Telescope in February-March 2016, we obtained high-resolution images of SNRs Tycho, W44 and IC443 that provided accurate integrated flux density measurements at 21.4 GHz: 8.8 ± 0.9 Jy for Tycho, 25 ± 3 Jy for W44 and 66 ± 7 Jy for IC443. We coupled the SRT measurements with radio data available in the literature in order to characterise the integrated and spatially-resolved spectra of these SNRs, and to find significant frequency-and region-dependent spectral slope variations. For the first time, we provide direct evidence of a spectral break in the radio spectral energy distribution of W44 at an exponential cutoff frequency of 15 ± 2 GHz. This result constrains the maximum energy of the accelerated electrons in the range 6 − 13 GeV, in agreement with predictions indirectly derived from AGILE and Fermi-LAT gammaray 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.

New high-frequency radio observations of the Cygnus Loop supernova remnant with the Italian radio telescopes

“…On the other hand, oversampling with respect to the beam size is required to perform a direct evaluation of statistical errors, and to properly reject the corrupted data affected by RFI. This leads to the achievement of improved accuracy in the final images, as demonstrated by our previous experience in single-dish imaging of Galactic sources (Egron et al 2016, Egron et al 2017, Loru et al 2019. For this reason, we performed four complete maps for each observing session, which allowed us to acquire ∼240 samples per beam once merged.…”

“…The receiver was used in the Best Space Coverage configuration (Bolli et al 2015) that automatically rotates the dewar to optimally cover the scanned area (scan spacing ∼0.87 arcmin, comparable to the half power beam width, HPBW). The observational strategy and data analysis procedure that is required to observe extended sources with this configuration is described in Loru et al (2019).…”

“…For this reason, we decided to separately observe two interesting regions of the Cygnus Loop: the northern-bright filament (NGC 6992) and the southern shell. We adopted the well-established OTF mapping strategy in use for the INAF single-dish radio telescope network (Egron et al 2017, Loru et al 2019. We decided to perform RA and Dec maps with different size, which are indicated respectively by the blue and orange boxes in Fig.…”

“…3. The multi-feed data at 18.7 and 24.8 GHz were calibrated considering each feed characterised by a specific efficiency (Orfei et al 2010;Loru et al 2019). For this purpose, we used calibrator maps of each SRT observing session in order to calculate the ratios between the expected flux of the calibrator and the peak counts related to each feed.…”

“…Furthermore, our measurement rules out any spectral steepening up to high radio frequencies, and it confirms the tendency suggested by the Planck data at 30 GHz. Although the Cygnus Loop is considered approximately at the same evolutionary phase of other SNRs that showed a synchrotron spectral break, as in the case of S 147 (Fürst & Reich 1986) and W44 (Loru et al 2019), or characteristic spectral features, as the spectral bump observed in the IC443 between 20 and 70 GHz (Onić et al 2017, Loru et al 2019, its integrated spectrum appears dominated by the non-thermal synchrotron emission without any noticeable deviation. This result could indicate an earlier evolutionary phase of this SNR, for which a spectral steepening is expected at higher frequencies, or the global effect of different electron populations across the remnant undergoing peculiar shocks conditions.…”