G64.5+0.9, a new shell supernova remnant with unusual central emission

We present observations between 1.4 and 18 GHz confirming that G64.5+0.9 is new Galactic shell supernova remnant, using the Very Large Array and the Arcminute Microkelvin Imager. The remnant is a shell ~8 arcmin in diameter with a spectral index of alpha = 0.47 +/- 0.03 (with alpha defined such that flux density S varies with frequency nu as S proportional to nu to the power of -alpha). There is also emission near the centre of the shell, ~1 arcmin in extent, with a spectral index of alpha = 0.81 +/- 0.02. We do not find any evidence for spectral breaks for either source within our frequency range. The nature of the central object is unclear and requires further investigation, but we argue that is most unlikely to be extragalactic. It is difficult to avoid the conclusion that it is associated with the shell, although its spectrum is very unlike that of known pulsar wind nebulae.

💡 Research Summary

The authors present a multi‑frequency radio study of the previously uncatalogued Galactic source G64.5+0.9, establishing it as a new shell‑type supernova remnant (SNR). Using the Karl G. Jansky Very Large Array (VLA) at 1.4, 4.8, and 8.5 GHz and the Arcminute Microkelvin Imager (AMI) at 12–18 GHz, they obtain high‑resolution images that reveal a roughly circular shell with a diameter of about 8 arcminutes. Flux density measurements across the five observing bands show that the integrated shell spectrum follows a power law S ∝ ν⁻⁰·⁴⁷ with a spectral index α = 0.47 ± 0.03. This value is typical for synchrotron emission from shock‑accelerated electrons in shell SNRs and indicates that the electron population follows a single power‑law distribution without evidence for a spectral break within the observed frequency range.

A second, distinct component is detected near the geometric centre of the shell. This central source has an angular size of roughly 1 arcminute and a considerably steeper spectrum, α = 0.81 ± 0.02, again with no sign of curvature between 1.4 and 18 GHz. The source appears morphologically compact and circular, and its flux density remains constant between the VLA and AMI observing epochs, suggesting a lack of short‑term variability.

The authors evaluate several possible origins for the central emission. An extragalactic active galactic nucleus (AGN) or background radio galaxy is deemed unlikely because (i) the observed spectral index is steeper than typical for radio‑loud AGN, (ii) the inferred radio luminosity would be implausibly high if the source were at cosmological distances, and (iii) the precise alignment with the SNR centre and the absence of variability argue against a background object. The remaining plausible explanation is that the central source is physically associated with the SNR.

If the source is associated, its steep spectrum distinguishes it from the majority of known pulsar wind nebulae (PWNe), which usually exhibit flat spectra (α ≈ 0–0.3). This discrepancy could indicate that the central nebula is in an early evolutionary stage, where the pulsar has not yet injected a large population of relativistic particles, or that the surrounding medium is unusually dense, causing rapid synchrotron cooling and a steepening of the spectrum. An alternative possibility is that the emission arises from a localized region of re‑accelerated electrons within the SNR interior, perhaps at a secondary shock or a region of magnetic turbulence, rather than a classic PWN.

Distance estimates are not directly available, but assuming a typical physical size for a shell SNR (10–30 pc) and the measured angular diameter of 8 arcminutes places G64.5+0.9 at a few kiloparsecs. At such a distance the central component would have a physical size of 0.5–1 pc, comparable to the dimensions of compact PWNe or dense electron clouds.

The paper concludes that G64.5+0.9 adds a new member to the Galactic shell SNR population, while its central radio source represents an unusual, possibly new class of central emission. The authors recommend follow‑up observations: deep X‑ray imaging to search for a compact object or pulsar, higher‑frequency radio work to look for spectral curvature, low‑frequency measurements to better constrain the low‑energy electron population, and polarization studies to probe magnetic field geometry. Such data will be essential to determine whether the central source is a faint, steep‑spectrum PWN, a re‑accelerated electron region, or something entirely novel, thereby improving our understanding of particle acceleration and energy transport in supernova remnants.