Astronomers have found dramatic evidence that a black hole or neutron star
spiraled its way into the core of a companion star and caused that companion
to explode as a supernova.
The astronomers were tipped off 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 had predicted that this could happen, but this is the first time
we've actually seen such an event," said Dillon Dong, a graduate student at
Caltech and lead author on a paper reporting the discovery in the journal
Science.
The first clue came when the scientists examined images from VLASS, which
began observations in 2017, and found an object brightly emitting radio
waves but which had not appeared in an earlier VLA sky survey, 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 bright radio emission was
coming from the outskirts of a dwarf, star-forming galaxy some 480 million
light-years from Earth. They later found that an instrument aboard the
International Space Station had detected a burst of X-rays coming from the
object in 2014.
The data from all these observations allowed the astronomers to piece
together the fascinating history of a centuries-long death dance between two
massive stars. Like most stars that are much more massive than our Sun,
these two were born as a binary pair, closely orbiting each other. One of
them was more massive than the other and evolved through its normal, nuclear
fusion-powered lifetime more quickly and exploded as a supernova, leaving
behind either a black hole or a superdense neutron star.
The black hole or neutron star's orbit grew steadily closer to its
companion, and about 300 years ago it entered the companion's atmosphere,
starting the death dance. At this point, the interaction began spraying 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 inward to the
companion star's core, disrupting the nuclear fusion producing the energy
that kept the core from collapsing of its own gravity. As the core
collapsed, it briefly formed a disk of material closely orbiting the
intruder and propelled a jet of material outward from the disk at speeds
approaching that of light, 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 date of this event in
2014," Dong said.
The collapse of the star's core caused it to explode as a supernova,
following its sibling's earlier explosion.
"The companion star was going to explode eventually, but this merger
accelerated the process," Dong said.
The material ejected by the 2014 supernova explosion moved much faster than
the material thrown off earlier from the companion star, and by the time
VLASS observed the object, the supernova blast was colliding with that
material, causing powerful shocks that produced the bright radio emission
seen by the VLA.
"All the pieces of this puzzle fit together to tell this amazing story,"
said Gregg Hallinan of Caltech. "The remnant of a star that exploded a long
time ago plunged into its companion, causing it, too, to explode," he added.
The key to the discovery, Hallinan said, was VLASS, which is imaging the
entire sky visible at the VLA's latitude -- about 80 percent of the sky --
three times over seven years. One of the objectives of doing VLASS that way
is to discover transient objects, such as supernova explosions, that emit
brightly at radio wavelengths. This supernova, caused by a stellar merger,
however, was a surprise.
"Of all the things we thought we would discover with VLASS, this was not one
of them," Hallinan said.
Reference:
A transient radio source consistent with a merger-triggered core collapse
supernova, SCIENCE, 3 Sep 2021, Vol 373, Issue 6559, pp. 1125-1129
DOI: 10.1126/science.abg6037
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