  Supernova remnants (SNRs) are observable for about 6-15x10^4 years before
they fade into the Galactic interstellar medium. With a Galactic supernova rate
of approximately two per century, we can expect to have of the order of 1200
SNRs in our Galaxy. However, only about 300 of them are known to date, with the
majority having been discovered in Galactic plane radio surveys. Given that
these SNRs represent the brightest tail of the distribution and are mostly
located 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-sky
survey data which led to the detection of one of the largest SNRs discovered at
wavelengths other than the radio: G249.5+24.5. This source is located at a
relatively high Galactic latitude, where SNRs are not usually expected to be
found. The remnant, 'Hoinga', has a diameter of about 4.4 degrees and shows a
circular shaped morphology with diffuse X-ray emission filling almost the
entire remnant. Spectral analysis of the remnant emission reveals that an APEC
spectrum from collisionally ionised diffuse gas and a plane-parallel shock
plasma model with non-equilibrium ionisation are both able to provide an
adequate description of the data, suggesting a gas temperature of the order of
kT = 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 identified
its 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 index
alpha=-0.69 +- 0.08 obtained from these data definitely confirms the SNR nature
of Hoinga. From its size and X-ray and radio spectral properties we conclude
that Hoinga is a middle-aged Vela-like SNR located at a distance of about twice
that of the Vela SNR, i.e. at ~500 pc.
