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

Abstract

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 gamma-ray emission, is likely associated to 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 detailed characterisation. We obtained new XMM-Newton observations, and produced count-rate images, equivalent width and median photon energy maps to identify optimal regions for spectral analysis. We complemented these observations with a reanalysis of Fermi/LAT gamma-ray data and new Telescopio Nazionale Galileo observations aimed to search for Halpha emission. The X-ray diffuse emission is well described by a model of shock-heated plasma with temperature kT ~ 0.15 keV, mildly under-solar N and O abundances and densities ne=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 hard emission equally well described by a thermal (kT ~ 1.7 keV) or a non thermal model (Gamma ~ 2.7). The brightest X-ray area is close to the gamma-ray peak and to an isolated Halpha filament. 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 gamma-ray emission and the detection of Halpha filaments indicates that a hadronic origin is compatible with the gamma-ray flux, though a mixed leptonic-hadronic cannot be excluded.