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Supernova remnants (SNRs) are observable for about 6-15x10^4 years beforethey fade into the Galactic interstellar medium. With a Galactic supernova rateof approximately two per century, we can expect to have of the order of 1200SNRs in our Galaxy. However, only about 300 of them are known to date, with themajority having been discovered in Galactic plane radio surveys. Given thatthese SNRs represent the brightest tail of the distribution and are mostlylocated close to the plane, they are not representative of the complete sample.Here we report findings from the search for new SNRs in the eROSITA all-skysurvey data which led to the detection of one of the largest SNRs discovered atwavelengths other than the radio: G249.5+24.5. This source is located at arelatively high Galactic latitude, where SNRs are not usually expected to befound. The remnant, 'Hoinga', has a diameter of about 4.4 degrees and shows acircular shaped morphology with diffuse X-ray emission filling almost theentire remnant. Spectral analysis of the remnant emission reveals that an APECspectrum from collisionally ionised diffuse gas and a plane-parallel shockplasma model with non-equilibrium ionisation are both able to provide anadequate description of the data, suggesting a gas temperature of the order ofkT = 0.1 keV and an absorbing column density of N_H=3.6 x 10^20 cm^-2.Subsequent searches for a radio counterpart of the Hoinga remnant identifiedits radio emission in archival data from the Continuum HI Parkes All-Sky Survey(CHIPASS) and the 408-MHz `Haslam' all-sky survey. The radio spectral indexalpha=-0.69 +- 0.08 obtained from these data definitely confirms the SNR natureof Hoinga. From its size and X-ray and radio spectral properties we concludethat Hoinga is a middle-aged Vela-like SNR located at a distance of about twicethat of the Vela SNR, i.e. at ~500 pc.

Hoinga: A supernova remnant discovered in the SRG/eROSITA All-Sky Survey eRASS1