How a black hole triggered a premature supernova

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The mystery of a particularly bright and short-lived radio signal that was picked up in 2017 seems to have been solved. It was probably a supernova that was triggered by a star collision.

Researchers at the California Institute of Technology (Caltech) in Pasadena, California are sureto have found an explanation for an extraordinary radio signal. They found this during a search in the data of the Very Large Array Telescope (VLA) in the US state of New Mexico. Its treasure trove of data is regularly searched for unusual radio signals. Especially so-called volatile radio sources, i.e. those that shine very briefly in the dark like a matchstick but very brightly, arouse the interest of science. And Dillon Dong from Caltech had discovered just such a source. It shone even brighter than any supernova – that is, the explosion in which a star passes – that had ever been discovered before. The VLA had cataloged the event as VT 1210 + 4956.

In his analyzes, Dong finds that the bright radio emission was originally a star that was surrounded by a thick and dense shell of gas. This gas envelope had been repelled by the star a few hundred years ago for an initially unexplained cause. VT 1210 + 4956, the extremely bright radio signal, must have been created when the star finally exploded in a supernova and the material ejected from the explosion collided with the gas envelope. However, the gas envelope itself and the length of time it was ejected from the star were unusual, leading Dong to suspect that there might be more to this explosion.

Dong and his colleagues were only able to find a plausible explanation for this extraordinarily strong supernova when the Caltech graduate student Anna Ho had the idea of ​​comparing VT 1210 + 4956 with another catalog of short, bright events in the X-ray spectrum. Some of the X-ray events recorded there were so short-lived that they could only be seen in the sky for a few seconds on Earth.

Indeed, Dong found what he was looking for while examining this other catalog. He was able to make out a source of x-rays that appeared to come from the same point in the sky as VT 1210 + 4956. Through careful analysis, Dong determined that the x-rays and radio waves must have come from the same event, but had not yet been linked.

After careful modeling, Dong and his team came up with a spectacular explanation. They suspected that a compact remnant of a previously exploded star – a black hole or a neutron star – had reached another star and then orbited tightly. In the course of time, the black hole had begun to suck up the star’s atmosphere and toss it into space, creating a ring-shaped bulge of gas. This process pulled the two objects closer and closer together until the black hole crashed into the star, which then collapsed and exploded as a supernova.

The X-rays, on the other hand, were generated by a beam that emerged from the core of the star at the moment of collapse. The radio waves, in turn, did not emerge until years later, when the exploding star reached the bulge of gas that had once been its own atmosphere.

In astronomy it is known that a massive star and a compact accompanying object can form a so-called stable orbit in which the two bodies spiral closer and closer together over an extremely long period of time. This creates a binary star system that can remain stable for millions to billions of years, but will ultimately end in a collision.

In the case of VT 1210 + 4956, however, the two objects collided much faster. VT 1210 + 4956 is considered to be the first evidence for the end of a binary star system, which so far had only been predicted in theory. The Caltech team published their research in the science journal Science on September 3rd.