Fermi-LAT detection of G118.4+37.0: a supernova remnant in the Galactic halo seen around the Calvera pulsar

TL;DR

This paper reports the detection of gamma-ray emission from the supernova remnant G118.4+37.0 near the Calvera pulsar, confirming its non-thermal nature and suggesting it as part of a potentially larger, radio-dim SNR population in low-density environments.

Contribution

First gamma-ray detection of the supernova remnant G118.4+37.0, linking radio and GeV emissions and proposing it as part of a new class of radio-dim SNRs.

Findings

Extended GeV emission aligns with the radio source.

Spectrum consistent with leptonic origin from relativistic electrons.

Supports the existence of a population of radio-dim SNRs in low-density environments.

Abstract

The discovery of a non-thermal radio ring of low surface brightness about one degree in diameter has been recently reported around the location in the sky of the Calvera pulsar, at a high Galactic latitude. The radio properties point to it likely being a new supernova remnant (SNR), G118.4+37.0. We report an analysis of almost 14 years of observations of this region by the gamma-ray Large Area Telescope onboard the Fermi satellite. We detect extended GeV emission consistent with the size and location of the radio source, which confirms the presence of relativistic particles. The spectrum of the high-energy emission is fully compatible with an origin in the same relativistic particles producing the radio emission. These features and its similarities to other isolated SNRs establish this source as the remnant of a supernova. A simple model of the non-thermal emission from radio to GeV energies resulting from leptonic emission from electrons produced by the SNR is presented. G118.4+37.0 and other similar isolated remnants could be part of a radio-dim SNR population evolving in low density environments showing hard GeV emission of leptonic origin. Future deeper surveys in radio and gamma-rays could discover new members of the group.