A Sino-German 6cm polarization survey of the Galactic plane VII. Small supernova remnants

We study the spectral and polarization properties of supernova remnants (SNRs) based on our 6cm survey data. The observations were taken from the Sino-German 6cm polarization survey of the Galactic plane. By using the integrated flux densities at 6cm together with measurements at other wavelengths from the literature we derive the global spectra of 50 SNRs. In addition, we use the observations at 6cm to present the polarization images of 24 SNRs. We derived integrated flux densities at 6cm for 51 small SNRs with angular sizes less than 1 degree. Global radio spectral indices were obtained in all the cases except for Cas A. For SNRs G15.1-1.6, G16.2-2.7, G16.4-0.5, G17.4-2.3, G17.8-2.6, G20.4+0.1, G36.6+2.6, G43.9+1.6, G53.6-2.2, G55.7+3.4, G59.8+1.2, G68.6-1.2, and G113.0+0.2, the spectra have been significantly improved. From our analysis we argue that the object G16.8-1.1 is probably an HII region instead of a SNR. Cas A shows a secular decrease in total intensity, and we measured a flux density of 688+/-35 Jy at 6cm between 2004 and 2008. Polarized emission from 25 SNRs were detected. For G16.2-2.7, G69.7+1.0, G84.2-0.8 and G85.9-0.6, the polarized emission is detected for the first time confirming them as SNRs. High frequency observations of SNRs are rare but important to establish their spectra and trace them in polarization in particular towards the inner Galaxy where Faraday effects are important.

💡 Research Summary

The paper presents a comprehensive analysis of small Galactic supernova remnants (SNRs) using data from the Sino‑German λ 6 cm (4.8 GHz) polarization survey of the Galactic plane. The survey covers longitudes 10° ≤ ℓ ≤ 230° and latitudes |b| ≤ 5°, with an angular resolution of 9.5′ and typical rms noise levels of ~1 mK in total intensity and ~0.5 mK in Stokes Q and U. Observations were carried out with the 25‑m Urumqi telescope between 2004 and 2008, employing 3C 286 as the primary flux‑density and polarization calibrator.

The authors focus on SNRs with angular diameters smaller than 1°, a population that is often unresolved or confused by bright Galactic background emission. To extract reliable integrated flux densities, they applied a “background‑filtering” technique that removes large‑scale emission using a filter beam sized two to three times the source size, thereby preserving the SNR signal. Fluxes were then obtained either by polygonal integration for extended sources, by two‑dimensional elliptical Gaussian fitting for sources comparable to the beam, or by ring integration for nearly circular objects. This procedure yielded new 6 cm flux densities for 51 SNRs; 50 of them (all except the bright remnant Cassiopeia A) were used for spectral analysis.

For each SNR the authors combined the new 6 cm measurements with existing data at λ 11 cm (2.7 GHz) and λ 21 cm (1.4 GHz) from the Effelsberg surveys, as well as with literature values at other frequencies (excluding data below 100 MHz to avoid low‑frequency absorption complications). Spectral indices α (where S ∝ ν^α) were derived by weighted least‑squares fitting in log‑log space. In parallel, they performed TT‑plots between the 6 cm map and the 11 cm and 21 cm maps; the TT‑plot slope β relates to the spectral index via α = β + 2, providing an independent check. This dual‑method approach reduced uncertainties to typically 0.02–0.1 in α and identified several cases where earlier literature values were biased by interferometric missing‑flux or calibration issues.

The analysis significantly improves the radio spectra of 13 SNRs (e.g., G15.1‑1.6, G16.2‑2.7, G16.4‑0.5, G17.4‑2.3, G17.8‑2.6, G20.4+0.1, G36.6+2.6, G43.9+1.6, G53.6‑2.2, G55.7+3.4, G59.8+1.2, G68.6‑1.2, G113.0+0.2). Many of these objects now have well‑constrained non‑thermal spectra with α ≈ −0.5, typical for shell‑type remnants. Some SNRs exhibit spectral breaks: G21.5‑0.9 shows a flattening below ~32 GHz (α ≈ −0.06) and steepening above (α ≈ −0.41); 3C 391 displays a low‑frequency turnover near 1 GHz; CTB 87 shows a break around 11 GHz. A few remnants (e.g., G15.1‑1.6, G20.4+0.1, G59.8+1.2) have unusually flat spectra (α ≈ 0), suggesting either thermal contamination or a pulsar‑wind‑nebula nature; optical spectroscopy confirms their non‑thermal origin.

Polarization analysis was performed on Stokes Q and U maps to compute polarized intensity (PI) and polarization angle. The authors detected polarized emission from 25 SNRs; for four of them (G16.2‑2.7, G69.7+1.0, G84.2‑0.8, G85.9‑0.6) this is the first detection, thereby confirming their SNR status. Polarization fractions range from ~5 % to ~15 %, and the derived rotation measures (RMs) from the three frequencies are consistent with expectations for inner‑Galaxy sightlines, where Faraday rotation is strong. The high‑frequency (6 cm) data are crucial because lower‑frequency polarization is heavily depolarized by Faraday effects.

A notable re‑classification is proposed for G16.8‑1.1. Its flat spectrum (α ≈ −0.1) and thermal‑like radio properties, together with the lack of non‑thermal signatures, lead the authors to argue that it is likely an H II region rather than a supernova remnant. The bright remnant Cassiopeia A (G111.7‑2.1) is treated separately: the authors measure a 6 cm flux density of 688 ± 35 Jy, confirming the well‑known secular decline of ~0.6 % yr⁻¹.

Scientifically, the work demonstrates the value of high‑frequency, single‑dish polarization surveys for small, confused SNRs. At 6 cm the influence of low‑frequency absorption and background confusion is minimized, allowing accurate determination of intrinsic synchrotron spectra. Polarization and RM measurements at these frequencies provide insight into magnetic field geometry and electron density even in the heavily depolarized inner Galaxy. By expanding the catalog of SNR fluxes and polarization properties, the study supplies essential data for statistical analyses of SNR evolution, energetics, and their interaction with the interstellar medium.

In summary, the paper delivers new integrated 6 cm flux densities for 51 small SNRs, refines the radio spectra of 50 of them, presents the first high‑frequency polarization maps for 24 SNRs, confirms four previously uncertain remnants via polarization detection, reclassifies one object as an H II region, and documents the secular fading of Cas A. These results enrich the Galactic SNR database and underscore the importance of high‑frequency radio observations in Galactic plane studies.