Scientists have tracked down the source of the strange radiation, which was first recorded four years ago. It turned out to be a collapsing star that turns into a black hole or neutron star.

In June 2018, telescopes around the world recorded a bright blue flash in an arm of a galaxy 200 million light-years from Earth. The researchers assumed it was a supernova, although it was much faster and brighter than any explosion that scientists have observed previously. The signal, which was named AT2018cow, was recorded as “an event of unknown origin.”

Now a team led by the Massachusetts Institute of Technology (MIT) has found compelling evidence for the signal’s source. In addition to the bright optical flare, the scientists detected a pulse of high-energy X-rays. They tracked hundreds of millions of these pulses and found that they occur every 4.4 milliseconds for 60 days.

Based on the pulse frequency, the team calculated that the X-rays should come from an object no more than 1,000 km wide and less than 800 solar masses. By astrophysical standards, such an object is considered compact – most likely a small black hole or a neutron star.

Their findings, published in the journal Nature Astronomy, suggest that AT2018cow was most likely the product of a dying star that collapsed into a compact object. It continued to absorb the surrounding material, eating away at the star from the inside – a process that resulted in a huge burst of energy.