Possible discovery of Calvera’s supernova remnant

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands
e-mail: arias@strw.leidenuniv.nl
² ASTRON Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
⁴ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁵ GEPI & USN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁶ Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands

Received: 28 June 2022
Accepted: 22 July 2022

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 α = 14h11m12^s.6, δ = +79°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 Hα band show no coincident optical emission, but they do show a small (~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.