Imaging of SNR IC443 and W44 with the Sardinia Radio Telescope at 1.5 and 7 GHz

Abstract

Observations of supernova remnants (SNRs) are a powerful tool for investigating the later stages of stellar evolution, the properties of the ambient interstellar medium, and the physics of particle acceleration and shocks. For a fraction of SNRs, multi-wavelength coverage from radio to ultrahigh-energies has been provided, constraining their contributions to the production of Galactic cosmic rays. Although radio emission is the most common identifier of SNRs and a prime probe for refining models, high-resolution images at frequencies above 5 GHz are surprisingly lacking, even for bright and well-known SNRs such as IC443 and W44. In the frameworks of the Astronomical Validation and Early Science Program with the 64-m single-dish Sardinia Radio Telescope, we provided, for the first time, single-dish deep imaging at 7 GHz of the IC443 and W44 complexes coupled with spatially-resolved spectra in the 1.5 − 7 GHz frequency range. Our images were obtained through on-the-fly mapping techniques, providing antenna beam oversampling and resulting in accurate continuum flux density measurements. The integrated flux densities associated with IC443 are S 1.5GHz = 134 ± 4 Jy and S 7GHz = 67 ± 3 Jy. For W44, we measured total flux densities of S 1.5GHz = 214 ± 6 Jy and S 7GHz = 94 ± 4 Jy. Spectral index maps provide evidence of a wide physical parameter scatter among different SNR regions: a flat spectrum is observed from the brightest SNR regions at the shock, while steeper spectral indices (up to ∼ 0.7) are observed in fainter cooling regions, disentangling in this way different populations and spectra of radio/gamma-ray-emitting electrons in these SNRs.

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

“…We estimate a spectral index of α = 1.26 ± 0.09 using the integrated flux density measurements carried out with SRT at 7.0 GHz and 21.4 GHz. A comparison between this result and that obtained by Egron et al (2017) α = 0.55 ± 0.03 (1.55−7.2 GHz) suggests a significant spectral index steepening at high frequencies.…”

“…The K-band flux density measured for W44 is far below the extrapolation of the spectrum obtained through low-frequency SRT measurements (Egron et al 2017). We estimate a spectral index of α = 1.26 ± 0.09 using the integrated flux density measurements carried out with SRT at 7.0 GHz and 21.4 GHz.…”

“…We performed the data analysis by using the "Single-Dish Imager" (SDI) software tool, which was tested during the SRT Commissioning and Scientific Validation (Bolli et al 2015, Prandoni et al 2017. SDI is designed to perform continuum and spectro-polarimetric multi-feed imaging starting from OTF scans, and is suitable for all SRT receivers/backends (see details in Egron et al 2016). The final SDI output are standard FITS images that can be used for further analysis with standard astronomy tools.…”

“…That is because these "mixed-morphology" sources offer a rich region-dependent phenomenology including atomic/molecular clouds interactions, Pulsar Wind Nebulae (PWN) associations, thermal X-ray emission, halos and filamentary structures (Seta et al 1998, Rho & Petre 1998. Recent single-dish imaging of W44 and IC443 was performed by Egron et al (2017) using the Sardinia Radio Telescope (SRT 1 ) data at 1.5 GHz and 7.0 GHz in the framework of the SRT astronomical validation (AV, Prandoni et al 2017) and the Early Science Program (ESP 2 ). For the first time, these observations provided deep single-dish imaging of W44 and IC443 at 7.0 GHz.…”

The flickering nuclear activity of Fornax A

“…The total flux density of Fornax A and the flux densities of the East and West lobes with errors are shown in Table 3. At each frequency, the error on the flux density is the combination between the noise in the image (shown in Table 2 for each observation) and the error due to the uncertainties in the calibration of the observations, which is 20% for MWA (McKinley et al 2015), 15% for Planck (Planck Collaboration et al 2018), 5% for SRT (Egron et al 2017;Battistelli et al 2019) and 3% for VLA (Perley & Butler 2017), respectively. For the MeerKAT observation we estimate a flux-calibration error of 5%, to allow for uncertainties in the flux of the gain calibrator.…”