A graduate student and a team of astronomers at Caltech discovered that an unusual source of radio waves observed in 2017 was caused by a black hole or neutron star colliding with a companion star, a press statement reveals.
Typically, large stars explode and become supernovae when they run out of fuel, bringing them crashing to the end of their lifespan. However, "in this case, an invading black hole or neutron star prematurely triggered its companion star to explode," according to Gregg Hallinan, professor of astronomy at Caltech. A paper published in the journal Science details how the discovery constitutes the first-ever confirmation of what they call a merger-triggered supernova.
The Caltech graduate student, Dillon Dong, was part of a team led by Hallinan, tasked with searching for short bursts of radio waves, also known as radio transients — as these are often signals for unusual astronomical events. Searching through data from the Very Large Array (VLA) Sky Survey, Dong brought a particularly strong radio transient, called VT 1210+4956, to the attention of the team. After more analysis, Dong and the team discovered that, even more unusually, X-rays listed in a separate survey were being emitted by the same event.
Investigating the death of a star
Dong explains that the Caltech team he is a part of set out to study the VLA Sky Survey with specific goals, while also keeping an open mind. Even so, they were surprised to discover such an unusual astronomical event. "The X-ray transient was an unusual event — it signaled that a relativistic jet was launched at the time of the explosion," Dong said. "And the luminous radio glow indicated that the material from that explosion later crashed into a massive torus of dense gas that had been ejected from the star centuries earlier. These two events have never been associated with each other, and on their own they're very rare."
The team speculated that a leftover remnant of a star that had previously exploded — either a black hole or a neutron star — had been orbiting near another star. The black hole gradually siphoned away the atmosphere of the nearby star, ejecting it into space and forming the torus of gas. Gradually, the stars were dragged closer together until the black hole plunged into the star, causing the star to collapse and form a supernova. The collapse caused the burst of X-rays while the radio transient was produced years later when the star reached the previously ejected torus of gas.
Such collisions have only been predicted in theory in the past, and Dong's new discovery provides the first real evidence that such a phenomenon can occur in the universe. The observation is yet another indicator that we still have so much to discover about the processes governing the death of stars. NASA's James Webb Telescope, which is mere months away from being shipped to its launch site, promises to provide vital data to astronomers like Dong and Hallinan, allowing them to fill gaps in our knowledge and to learn more about the origin and demise of stars like the one at the center of our solar system.