Possible discovery of Calvera's 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 $\hat{I}\pm =14\mathrm{h}11\mathrm{m}12.6\mathrm{s}$, $\hat{I}´=+79^{\mathrm{o}}23^{\prime }15"$, has inner and outer radii of $14.2^{\prime }$ and $28.4^{\prime }$, and an integrated flux density at 144 MHz of $1.08\pm 0.15$ Jy. The ring center is offset by $4.9^{\prime }$ from the location of the Calvera pulsar. H$\hat{I}\pm$ observations with the Isaac Newton Telescope show no coincident optical emission, but do show a small ($\sim 20"$) 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.

Accepted in A&A