Last June, astronomers in Hawaii saw a bright, but brief, burst of light emanating from a galaxy named CGCG 137-068. They believe that they witnessed the birth of a black hole, or perhaps a neutron star.
Initial observations suggested that it was a supernova, however the explosion was ten to 100 times brighter than that of a typical supernova. The light generated from the event (which has been named AT2018cow, or “The Cow”) was so bright that for a brief moment, it was brighter than its host galaxy.
According to Space.com:
“Based on its X-ray and UV [ultraviolet] emission, ‘The Cow’ may appear to have been caused by a black hole devouring a white dwarf,” study lead author Raffaella Margutti, an assistant professor of physics and astronomy at Northwestern University in Illinois, said in a statement.
“But further observations of other wavelengths across the spectrum led to our interpretation that ‘The Cow’ is actually the formation of an accreting black hole or neutron star,” Margutti added. “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.”
The event took place 200 million light years away, and for those of us on Earth, appeared in the Hercules constellation.
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A black hole is formed when an enormous amount of matter is packed into a relatively small area. The resulting gravitational pull is so intense that not even light can escape. A neutron star is the collapsed core of a star between ten and 29 times the size of our sun. They generally have a diameter around 20 kilometers, however their mass can be approximately twice as much as our sun.
Others have pointed out that unlike normal supernovas, The Cow did not have a gradual fade, and instead had “bumps and wiggles” in brightness, leading them to believe that there may be a “central engine” that is still pumping energy into the remnants.
At later times, as the Cow was fading, the GROWTH team observed subtle bumps and wiggles in the optical data rather than the smooth decline in brightness that is more typical for supernovae. According to Perley, this suggests that there is an additional source of power, or “central engine” that keeps pumping energy into the expanding material.
Half a year after the Cow was first spotted, astronomers continue to debate what it is and how it really happened. The most favored interpretation, supported by the GROWTH team, is that the Cow is an exotic type of supernova that left behind a magnetar central engine. Other teams suggest that a black hole ripping apart a type of star called a white dwarf may have instead caused it.
(A magnetar is a type of neutron star that has an extremely powerful magnetic field.)
While it’s not certain what exactly happened, we do know that it was something big. And it goes to show that as much as we know about the world and universe we live in, we still don’t know very much.
The paper will be published in The Astrophysical Journal.