Stellar collision triggers supernova explosion

Supernova explosions are fast-moving particles that collide with material from earlier collisions. The shocks produce bright radio emission which is visible by the VLA. Credit: Bill Saxton, NRAO/AUI/NSF
Astronomers found evidence that a black or neutron star spiraled into the core of another star, causing that star to explode as supernova. Data from the Very Large Array Sky Survey, a multiyear project that used the National Science Foundation's Karl G. Jansky Very Large Array, alerted the astronomers.

"Theorists predicted this would happen, but we haven't seen it yet," stated Dillon Dong (Caltech graduate student) who was the lead author of a Science paper that reported the discovery.

Scientists discovered the first clue when they examined VLASS images, which were made in 2017. They found a brightly emitting object that was not visible in an earlier VLA sky scan, Faint Images of the Radio Sky At Twenty Centimeters (FIRST). The object was later identified as VT 1210+4956 by the scientists. They observed it using both the VLA telescope in Hawaii and the Keck telescope. The bright radio emission they detected was coming from the edge of a star-forming dwarf galaxy approximately 480 million light years from Earth. Later, they discovered that the International Space Station's instrument had detected a burst in X-rays from the object in 2014.

All these observations enabled astronomers to reconstruct the fascinating story of a centuries-long death and resurrection dance between two huge stars. These stars, like most other massive stars, were formed as a binary pair. They orbited each other closely. One was larger than the other, and it lived a normal nuclear fusion-powered life faster. It exploded as either a supernova or a black hole, leaving behind either an extremely dense neutron star or a superdense neutron.

The orbit of the black hole or neutron star grew closer to its companion. It entered the atmosphere around 300 years ago, triggering the death dance. The interaction started to spray gas from the companion into space at this point. The spiralling outwards of the ejected gas formed a donut-shaped, expanding ring called a torus around the pair.

The black hole, or neutron star, eventually made its way into the companion star's center. It disrupted the nuclear fusion and produced the energy that prevented the core from collapsing under its own gravity. The core fell apart, forming a disk of material close to the intruder. This propelled a jet from the disk at speeds similar to light and drilling its way through star.

Dong stated, "That jet produced the Xrays seen by MAXI instrument aboard International Space Station and this confirms that this event took place in 2014."

After its sister's earlier explosion, the star's core collapsed and it exploded as a supernova.

Dong stated that "the companion star was going eventually to explode, but this merger accelerated it."

The 2014 supernova explosion produced material that moved faster than that of the companion star. By the time VLASS saw the object, the supernova blast had collided with the material and caused powerful shocks, which led to the VLA's bright radio emission.

Gregg Hallinan, Caltech, said that "all the pieces of this puzzle fit together and tell this incredible story." He added that the remnant of a star which exploded long ago plunged into its companion, causing it to also explode.

Hallinan stated that VLASS was the key to the discovery. It is a system that images the entire sky at the VLA's latitudeabout 80% of the skythree-times over seven years. VLASS is a way to find transient objects such as supernova blasts that emit at radio wavelengths. The supernova was caused by a stellar merging, but it was unexpected.

Hallinan stated, "Of all things we thought would be discovered with VLASS,"

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Further information: D. Z. Dong et., A transient radio signal consistent with a merger-triggered supernova, Science (2021). Information from Science D. Z. Dong and colleagues, A transient radiosource consistent with a merger triggered core collapse supernova (2021). DOI: 10.1126/science.abg6037