AT2020mrf could have remained one of many hundreds of ordinary optical transients that are regularly detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS). However, in the same place in the sky, and at about same time, the eROSITA telescope aboard the SRG orbital X-ray observatory, discovered an unusual new X-ray source, called SRGe J154754.2+443907. It soon became clear that scientists had found a unique cosmic object. They witnessed the birth of a new magnetar — a neutron star with the superstrong magnetic field – or of a black hole in a distant galaxy.
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