Possible discovery of Calvera's supernova remnant

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 = 14h11m12(s).6, delta = +79 degrees 23 ' 15 '', has inner and outer radii of 14 '.2 and 28 '.4, and has an integrated flux density at 144 MHz of 1.08 +/- 0.15 Jy. The ring center is offset by 4 '.9 from the location of the Calvera pulsar. Observations with the Isaac Newton Telescope in the H alpha band show no coincident optical emission, but they do show a small (similar to 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 ionization plasma previously detected lead us to prefer the SNR interpretation. If the source is indeed a SNR and if 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.

Automated summary

We report the discovery of a ring of low surface brightness radio emission around the Calvera pulsar, indicating a possible supernova remnant. The ring shows positional coincidence with the pulsar and an X-ray-emitting non-equilibrium ionization plasma, suggesting its association with the supernova remnant interpretation.