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7th of May 2021
A newly discovered supernova remnant
by Becker et al.
Supernova remnants (SNRs) are observable for about 100,000 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 radio surveys of the Galactic plane. 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. The launch of the Russian-German X-ray satellite SRG/eROSITA in July 2019 brought a new opportunity to search for SNRs (amongst many other things!). Becker et al. have recently reported the eROSITA 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. (Hoinga is named in honor of Becker's hometown Bad Hönningen am Rhein: Hoinga was its medieval name.)

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 all-sky survey of Haslam et al. The radio spectral index of α = −0.69 ± 0.08 obtained from these data confirms the SNR nature of Hoinga. The image shows the 10 degree x 10 degree region surrounding Hoinga as seen at 1.4 GHz by CHIPASS. The left panel shows the original image, and the right panel shows the image after point-like sources have been subtracted. Hoinga is clearly visible as a crescent-moon in the centre of the image, while "Galactic cirrus" and residuals around poorly subtracted diffuse sources are visible in the surroundings. The bright source in the northwest is the bright and extended radio galaxy Hydra A. Faint scan lines are visible from the Parkes observing strategy.

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