Astronomers have discovered compelling evidence that a black hole or neutron star spiraled its way into the core of a companion star, causing it to explode as a supernova. Data from the Very Large Array Sky Survey (VLASS), a multi-year project using the National Science Foundation’s Karl G. Jansky Very Large Array, tipped the astronomers off (VLA).
“Theorists predicted this could happen, but this is the first time we’ve actually seen such an event,” said Dillon Dong, a Caltech graduate student and lead author on a paper reporting the discovery published in the journal Science.
The first hint came when the scientists examined images from VLASS, which began observations in 2017, and discovered an object emitting radio waves but not visible in an earlier VLA sky survey called Faint Images of the Radio Sky at Twenty Centimeters (FIRST). They continued to observe the object, known as VT 1210+4956, with the VLA and the Keck telescope in Hawaii. They discovered that the bright radio emission was coming from the outskirts of a dwarf star-forming galaxy 480 million light-years away from Earth. They later discovered that in 2014, an instrument aboard the International Space Station detected a burst of X-rays coming from the object.
The Very Large Array Sky Survey gave astronomers the first clue that ultimately revealed a dramatic story, the remnant of a star that exploded long ago had plunged into the core of its companion star causing it, too, to explode as a supernova.
The astronomers were able to piece together the fascinating history of a centuries-long death dance between two massive stars thanks to the data from all of these observations. These two, like most stars much more massive than our Sun, were born as a binary pair, closely orbiting each other. One was more massive than the other and evolved faster through its normal, nuclear fusion-powered lifetime before exploding as a supernova, leaving behind either a black hole or a superdense neutron star.
The orbit of the black hole or neutron star grew steadily closer to that of its companion, and around 300 years ago it entered the companion’s atmosphere, beginning the death dance. At this point, the interaction began spraying gas into space away from the companion. The ejected gas spiraled outward, forming an expanding, donut-shaped ring around the pair known as a torus.
The black hole or neutron star eventually made its way into the companion star’s core, disrupting the nuclear fusion that produced the energy that kept the core from collapsing due to its own gravity. As the core collapsed, it briefly formed a disk of material orbiting the intruder and propelled a jet of material outward from the disk at near-light speeds, drilling its way through the star.
“That jet is what produced the X-rays seen by the MAXI instrument aboard the International Space Station, and this confirms the 2014 date,” Dong said. Following its sibling’s earlier explosion, the collapse of the star’s core caused it to explode as a supernova. “The companion star was going to explode eventually,” Dong explained, “but this merger has accelerated the process.”
The material ejected by the 2014 supernova explosion moved much faster than the material ejected earlier from the companion star, and by the time VLASS observed the object, the supernova blast had collided with that material, causing powerful shocks that produced the VLA’s bright radio emission.
“All of the pieces of this puzzle fit together to tell this amazing story,” said Caltech’s Gregg Hallinan. “A remnant of a star that exploded a long time ago collided with its companion, causing it to explode as well,” he added.
The key to the discovery, according to Hallinan, was VLASS, which is imaging the entire sky visible at the VLA’s latitude about 80 percent of the time over a seven-year period. One goal of doing VLASS in this manner is to find transient objects, such as supernova explosions, that emit brightly at radio wavelengths. This supernova, however, was unexpected. It was caused by a stellar merger.
“This was not one of the things we expected to discover with VLASS,” Hallinan said. The National Radio Astronomy Observatory is a National Science Foundation facility run by Associated Universities, Inc. under a cooperative agreement.
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