In June 2018 astronomers were bewildered by an unusually bright, but also very short, supernova. Given the designation AT2018cow, it became known as The Cow. Evidence has now been presented suggesting we witnessed the origins of either a neutron star or a black hole. Both types of object can be produced by an ordinary supernova, but this was definitely something special.
Supernovas are astonishingly rapid in astronomical terms, rising to their peak in two to three weeks, and trailing off over months. The Cow was faster still, peaking within days.
“That was enough to get everybody excited because it was so unusual,” said Dr Raffaella Margutti of Northwestern University in a statement. Within 16 days The Cow had faded almost to optical invisibility.
On cosmological scales The Cow was relatively close, happening in a dwarf galaxy a mere 200 million light-years away. Close enough for us to detect particles being expelled from its vicinity at 10 percent of the speed of light.
In recent times we have witnessed the merging of two neutron stars in a variety of devices, and we've also detected the gravitational waves from black holes colliding. An early hypothesis for The Cow was that it represented something like that, but Margutti found traces of hydrogen and helium in the afterglow, eliminating that explanation since such events would produce heavier elements instead.
Margutti proposed The Cow represented the formation of an immensely dense object at the Seattle meeting of the American Astronomical Association. “We know from theory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born. Never,” she said. She thinks the light we saw was the gas from the star's outer layers swirling around, possibly drawn towards a black hole's event horizon or caught in a highly magnetized neutron star's spin.
Margutti's conclusions are drawn from having studied the event on telescopes that collect electromagnetic radiation across the spectrum from radio waves to gamma rays. AT2018cow has been described as possibly the most observed astronomical event ever. At some wavelengths, it remained visible long after it had faded from view in the optical part of the spectrum.
Despite the evidence Margutti provided, some attendees remain open to the theory we saw a black hole eating a star.
The formation of black holes and neutron stars is usually obscured by large amounts of material thrown off in the stellar explosion. This bovine was naked, however, with 10 times less ejecta than normal.
Events that may have resembled The Cow have been seen before, but they were at greater distances, and we didn't study them at the range of wavelengths Margutti managed, leaving us with less of an idea of what went on.