In June 2018, a bright light burst into the skies over the Northern Hemisphere. At a glance it looked like any other supernova, but on closer inspection this thing turned out to be far weirder. Officially known as AT2018cow (but quickly nicknamed "The Cow"), astronomers now believe the ATLAS survey's twin telescopes in Hawaii captured an unprecedented look at the birth of a black hole or a neutron star.
When stars die, they put on some impressive displays of celestial fireworks, before collapsing into either black holes or neutron stars. The light brightens and then glows for months or years at a time, before fading away, leaving the new object in its wake. Although this happens all the time, astronomers usually miss the start of the show, and the resulting black hole or neutron star stays hidden in the dusty aftermath.
But The Cow was different. For one, it was much brighter than usual, shining up to 100 times brighter than your garden variety supernova. And as suddenly as it appeared, it was gone, fading away almost entirely in just 16 days, rather than months.
"We thought it must be a supernova," says Raffaella Margutti, lead researcher on the study. "But what we observed challenged our current notions of stellar death. We think that 'The Cow' is the formation of an accreting black hole or neutron star. 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."
Other factors also made The Cow particularly favorable viewing. At a distance of 200 million light-years it was relatively close to Earth, cosmically speaking, and with far less material swirling around than usual, the team could see right through to the object in the middle.
"A 'lightbulb' was sitting deep inside the ejecta of the explosion," says Margutti. "It would have been hard to see this in a normal stellar explosion. But The Cow had very little ejecta mass, which allowed us to view the central engine's radiation directly."
The team observed the object in X-rays, hard X-rays, radio waves and gamma rays, and concluded that the initial brightness was caused by stellar debris falling into the event horizon. When the researchers examined its chemical composition, they found evidence of hydrogen and helium, which rules out the possibility of the explosion being the result of two objects colliding.
As unique an event as it is, The Cow might not be the first time astronomers have witnessed the birth of a black hole. In 2017 a team reported that a star just disappeared before their eyes – no explosion, just a quiet collapse into a black hole. Finding anomalous events like this and The Cow can help fill out our understanding of the life and death of stars and black holes.
The research will be published in the Astrophysical Journal.
Source: Northwestern University
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