Earth-sized telescope spots star being swallowed by black hole

Scientists have used a radio telescope network the size of the Earth to zoom in on a unique phenomenon in a distant galaxy - a jet activated by a star being consumed by a supermassive black hole.

The record-sharp observations show a compact and slow-moving source of radio waves, researchers said.

Researchers, led by Jun Yang from Onsala Space Observatory at Chalmers University of Technology in Sweden, studied the new-born jet in a source known as Swift J1644+57 with the European VLBI Network (EVN), an Earth-size radio telescope array.

When a star moves close to a supermassive black hole it can be disrupted violently. About half of the gas in the star is drawn towards the black hole and forms a disc around it.

During this process, large amounts of gravitational energy are converted into electromagnetic radiation, creating a bright source visible at many different wavelengths.

One dramatic consequence is that some of the star's material, stripped from the star and collected around the black hole, can be ejected in extremely narrow beams of particles at speeds approaching the speed of light.

These so-called relativistic jets produce strong emission at radio wavelengths.

The first known tidal disruption event that formed a relativistic jet was discovered in 2011 by the NASA satellite Swift.

Initially identified by a bright flare in X-rays, the event was given the name Swift J1644+57. The source was traced to a distant galaxy, so far away that its light took around 3.9 billion years to reach Earth.

Researchers used the technique of very long baseline interferometry (VLBI), where a network of detectors separated by thousands of kilometres are combined into a single observatory, to make extremely high-precision measurements of the jet from Swift J1644+57.

"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-Euro coin on the Moon as seen from Earth. These are some of the sharpest measurements ever made by radio telescopes," said Yang.

"We looked for motion close to the light speed in the jet, so-called superluminal motion. But our images reveal instead very compact and steady emission - there is no apparent motion," said Yang.

The results give important insights into what happens when a star is destroyed by a supermassive black hole, but also how newly launched jets behave in a pristine environment.

The study was published in the journal Monthly Notices of the Royal Astronomical Society.