Astronomers have captured convincing evidence of a rare double cosmic cannibalism — a star consuming a compact object such as a black hole or neutron star — for the first time. As a result, the item devoured the star’s core, causing it to explode and leave only a black hole behind.
The Very Large Array (VLA), a radio telescope consisting of 27 large dishes in the New Mexican desert near Socorro, provided the first hints of the gruesome event, which was detailed in Science. During a scan of the night sky by the observatory in 2017, a blast of radio energy as bright as the brightest exploding star — or supernova — observed from Earth emerged in a dwarf star-forming galaxy 500 million light-years away.
“We thought, ‘Whoa, this is interesting,'” recalls Caltech astronomer Dillon Dong.
He and his colleagues used the VLA and one of the telescopes at the W.M. Keck Observatory in Hawaii to observe the galaxy in the same optical light as our eyes. A brilliant outflow of material erupting in all directions at 3.2 million kilometers per hour from a central point was captured by the Keck telescope, indicating that an intense explosion had occurred there in the past.
The team then found an extremely bright X-ray source in archival data from the Monitor of All Sky X-ray Image (MAXI) telescope, a Japanese instrument that sits on the International Space Station. This X-ray burst occurred in the same location as the radio burst, however it was observed in 2014.
Dong and his colleagues believe the following happened after piecing together the data: a binary pair of stars orbited each other; one died in a spectacular supernova explosion and became either a neutron star or a black hole. The dead star actually entered the outer layers of its larger stellar brother as gravity pulled the two objects closer together.
For hundreds of years, the compact object spiraled inside the still-living star, eventually descending to and devouring its partner’s core. During this time, the larger star shed massive amounts of gas and dust, building a material shell around the pair.
Gravitational forces and complex magnetic interactions from the dead star’s munching released massive jets of energy, which were detected as an X-ray burst in 2014, as well as causing the larger star to explode. The detonation debris crashed with massive force into the surrounding shell of material, generating optical and radio light.
While theorists have previously envisioned such a scenario, dubbed a merger-triggered core collapse supernova, this appears to represent the first direct observation of this phenomenon, Dong says.
“They’ve done some pretty good detective work using these observations,” says Adam Burrows, an astrophysicist at Princeton University who was not involved in the new study. He believes the discoveries will help constrain the timing of a process known as common envelope evolution, in which one star becomes swallowed by another. Such stages in the lives of stars are quite short in cosmic time and difficult to detect and simulate. Most of the time, the engulfing partner dies before its core is consumed, leading to two compact objects like white dwarfs, neutron stars or black holes orbiting one another.
According to Dong, the latter stages of these systems are exactly what observatories like the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, detect when collecting spacetime’s ripples. He expects astronomers to find more examples of this peculiar phenomenon now that they know to look for these various lines of evidence.