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The rapidly moving debris from a supernova explosion triggered by a stellar collision crash into the gas released earlier, and the shocks cause a luminous radio emission seen by the VLA. Credit: Bill Saxton, NRAO / AUI / NSF
Astronomers have found dramatic evidence that a black hole Where neutron star made its way into the core of a companion star and detonated that companion into a supernova. Astronomers were warned by 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 (VLA).
“Theorists predicted it could happen, but this is the first time we’ve seen such an event,” said Dillon Dong, a graduate student at Caltech and lead author of an article reporting the discovery in the journal. Science.
The first clue came when scientists looked at images from VLASS, which began observations in 2017, and found an object emitting bright radio waves but which had not appeared in a previous study of the VLA sky, called Faint Images of the Radio Sky at Twenty Centimeters (FIRST). They made subsequent observations of the object, designated VT 1210 + 4956, using the VLA and the Keck telescope in Hawaii. They determined that the light radio emission came from the outskirts of a dwarf galaxy forming stars some 480 million light years from Earth. They later discovered that an instrument aboard the International Space Station detected a burst of x-rays from the object in 2014.
The data from all of these observations have allowed astronomers to piece together the fascinating story of a centuries-long dance of death between two massive stars. Like most stars much more massive than our Sun, these two stars were born as a binary pair, orbiting each other closely. One of them was more massive than the other and evolved faster throughout its normal lifespan, fueled by nuclear fusion, and exploded as a supernova, leaving behind either a black hole, or a superdense neutron star.
Sequence of Events – Clockwise from top left: (1.) A neutron star or black hole orbits a ‘normal’ (light blue) companion star, bringing together for thousands of years. (2.) The neutron star or black hole enters its companion’s atmosphere, projecting gas outward in an expanding spiral. (3.) When the intruder reaches the core of the companion, the material briefly forms a disc that propels a super-fast jet outward, pushing its way out of the star. The nuclear fusion that held the companion’s core against its own gravity is disrupted, triggering a collapse and subsequent supernova explosion. (4.) The material thrown by the supernova explosion catches up with the material thrown by the previous interaction, causing strong shock waves that produce the radio waves observed with the VLA. Credit: Bill Saxton, NRAO / AUI / NSF
The orbit of the black hole or neutron star gradually moved closer to its mate, and about 300 years ago it entered the mate’s atmosphere, starting the dance of death. At this point, the interaction began to project gas away from the companion into space. The ejected gas, spiraling outward, formed an expanding donut-shaped ring, called a torus, around the pair.
Eventually, the black hole or neutron star made its way to the core of the companion star, disrupting nuclear fusion producing the energy that kept the core from collapsing by its own gravity. When the core collapsed, it briefly formed a disc of matter in a narrow orbit around the intruder and propelled a jet of matter out of the disc at speeds approaching that of light, forcing its way through through the star.
“This jet is what produced the x-rays seen by the MAXI instrument aboard the International Space Station, and it confirms the date of this event in 2014,” Dong said.
The star’s core collapse caused it to explode into a supernova, following its brother’s previous explosion.
“The companion star was going to explode eventually, but this merger sped up the process,” Dong said.
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 explosion was in collision with this material, causing powerful shocks which produced the light radio emission seen by the VLA.
“All the pieces of this puzzle come together to tell this incredible story,” said Gregg Hallinan of Caltech. “The remnant of a star that exploded long ago has plunged into his mate, causing her to explode too,” he added.
The key to the discovery, Hallinan said, was VLASS, which images the entire visible sky at the latitude of the VLA – about 80% of the sky – three times in seven years. One of the goals of this VLASS method is to discover transient objects, such as supernova explosions, that emit brightly at radio wavelengths. This supernova, caused by a stellar merger, however, came as a surprise.
“Of all the things we thought we would discover with VLASS, this was not one of them,” said Hallinan.
Reference: “A transient radio source coherent with a nucleus collapse supernova triggered by a fusion” September 2, 2021, Science.
DOI: 10.1126 / science.abg6037
The National Radio Astronomical Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.
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