Possible discovery of Calvera's supernova remnant

TL;DR

This paper reports the discovery of a radio emission ring near the Calvera pulsar, likely a supernova remnant, which could be one of the few in the Galactic halo, based on multi-wavelength observations.

Contribution

The discovery of a new radio ring structure around Calvera, with evidence suggesting it is a supernova remnant, expanding knowledge of SNRs in the Galactic halo.

Findings

Radio ring centered near Calvera pulsar with specific size and flux.

Optical observations show no emission, but a small internal structure.

Positional coincidence suggests the ring is a supernova remnant.

Abstract

We report the discovery of a ring of low surface brightness radio emission around the Calvera pulsar, a high Galactic latitude, isolated neutron star, in the LOFAR Two-metre Sky Survey (LoTSS). It is centered at $\alpha=14\mathrm{h}11\mathrm{m}12.6\mathrm{s}$, $\delta=+79^\mathrm{o}23'15"$, has inner and outer radii of $14.2'$ and $28.4'$, and an integrated flux density at 144 MHz of $1.08\pm0.15$ Jy. The ring center is offset by $4.9'$ from the location of the Calvera pulsar. H$\alpha$ observations with the Isaac Newton Telescope show no coincident optical emission, but do show a small ($\sim20"$) optical structure internal to the ring. We consider three possible interpretations for the ring: that it is an H~II region, a supernova remnant (SNR), or an Odd Radio Circle (ORC). The positional coincidence of the ring, the pulsar, and an X-ray-emitting non-equilibrium ionisation plasma previously detected, lead us to prefer the SNR interpretation. If the source is indeed a SNR and its association with the Calvera pulsar is confirmed, then Calvera's SNR, or G118.4+37.0, will be one of few SNRs in the Galactic halo.