Staring at someone while they’re eating is very rude, but for radio astronomers, it’s the only way to learn about the life of supermassive black holes.
An international team, led by Jun Yang from Onsala Space Observatory in Sweden, used a network of telescopes to study the jet activated by a star being eaten by a supermassive black hole, located 3.9 billion light-years from Earth.
This is the first-known detection of a tidal disruption event, the technical term for a star being torn apart, with the emission of a jet of particles moving at relativistic speed. These observations were possible thanks to a technique called very long baseline interferometry (VLBI), which effectively turns all the radio telescopes from the European VLBI network (EVN) into a single instrument the size of Earth.
"Using the EVN telescope network we were able to measure the jet's position to a precision of 10 microarcseconds. That corresponds to the angular extent of a €2 coin [2.6 centimeres/1 inch] on the Moon as seen from Earth. These are some of the sharpest measurements ever made by radio telescopes," said Jun Yang in a statement.
The study, published in the Monthly Notices of the Royal Astronomical Society, shows that the jet is not being shot out at the speed of light but actually remains quite compact, and it might be due to the interaction between the jet and the dense material around the black hole.
"We looked for motion close to the light speed in the jet, so-called superluminal motion," added Jun Yang. "Over our three years of observations, such movement should have been clearly detectable. But our images reveal instead very compact and steady emission – there is no apparent motion."
The researchers are confident that the jet is moving at less than one-third of the speed of light, which is very slow when it comes to black hole emissions.
"Newly formed relativistic ejecta decelerate quickly as they interact with the interstellar medium in the galaxy," said Jun Yang. "Besides, earlier studies suggest we may be seeing the jet at a very small angle. That could contribute to the apparent compactness."
This result provides scientists with an important insight in the aftermath of a black hole feeding, both in terms of how a star is destroyed and how jets interact with the untouched environment around a galactic core.