Multi-wavelength study of the high Galactic latitude supernova remnant candidate G118.4+37.0 associated with the Calvera pulsar

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via Corti 12, 20133 Milano, Italy
⁴ Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo, Via Archirafi 36, 90123 Palermo, Italy

^★ Corresponding author: emanuele.greco@inaf.it

Received: 23 May 2025
Accepted: 17 July 2025

Abstract

Context. The candidate supernova remnant (SNR) G118.4+37.0 (Calvera’s SNR), discovered as a faint radio ring at high Galactic latitude and coincident with extended Fermi/LAT γ-ray emission, is likely associated with the X-ray pulsar 1RXS J141256.0+792204 (Calvera). Previous XMM-Newton data hinted at soft diffuse X-ray emission inside the ring, but lacked sufficient exposure for a detailed characterization.

Aims. We aim to determine the distance, age, and physical conditions of Calvera’s SNR and establish whether its γ-ray emission is dominated by hadronic or leptonic processes.

Methods. We obtained new XMM-Newton observations and produced count-rate images, along with equivalent width and median photon energy maps to identify optimal regions for spectral analysis. We complemented these observations with a reanalysis of Fermi/LAT γ-ray data and new Telescopio Nazionale Galileo observations aimed at searching for Hα emission.

Results. The X-ray diffuse emission is well described by a model of shock-heated plasma with a temperature of kT ~ 0.15 keV, mildly under-solar N and O abundances, and densities n_e =0.1–0.7 cm^–3. According to our estimates, Calvera’s SNR is 10–20 kyr old and lies at a distance of 4–5 kpc. A distinct clump region shows harder emission equally well described by a thermal (kT ~ 1.7 keV) or a nonthermal model (Γ ~ 2.7). The brightest X-ray area is close to the γ-ray peak and an isolated Hα filament.

Conclusions. G118.4+37.0 is a middle–aged remnant which expands in a tenuous medium and encountered a denser phase, likely the relic of the wind activity of the massive progenitor star. The estimated SNR distance is consistent within the uncertainties with that estimated for Calvera, confirming that this peculiar pulsar was born in the explosion of a massive star high above the Galactic disk. Our measured ambient density, together with the patchy morphology of the γ-ray emission and the detection of Hα filaments indicate that a hadronic origin is compatible with the γ-ray flux; although a mixed leptonic–hadronic scenario cannot be excluded.