An international team of astronomers led by researchers at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching (Germany) has discovered powerful infrared flares from the supermassive black hole at the heart of the Milky Way.

The signals, rapidly flickering on a scale of minutes, must come from hot gas falling into the black hole, just before it disappears below the "event horizon" of the monster. The new observations strongly suggest that the Galactic Centre black hole rotates rapidly.

Never before have scientists been able to study phenomena in the immediate neighbourhood of a black hole in such a detail. The new result is based on observations obtained with the NACO Adaptive Optics instrument on the 8.2-m VLT KUEYEN telescope and is published in this week's edition of the research journal Nature.

Flashes of light from disappearing matter
The scene was the usual one in the VLT Control Room at the Paranal Observatory in the early morning of May 9, 2003. Groups of astronomers from different nations were sitting in front of the computer screens, pointing the four giant telescopes in different directions and recording the sparse photons from the remotest corners of the Universe. There were the usual brief exchanges of information, numbers, wavelengths, strange acronyms, but then suddenly something happened at the YEPUN desk....

The detection of a powerful flare from the center of the Milky Way galaxy. Credit: ESO

"What is that star doing there?" exclaimed Rainer Schodel, one of the MPE scientists in the team working with the NACO Adaptive Optics instrument that delivers razor-sharp images. He and Reinhard Genzel, leader of the team and MPE Director, were observing the Milky Way Centre, when they saw the "new" object on the screen in front of them. The astronomers were puzzled and then became excited - something unusual must be going on, there at the centre of our galaxy!

And then, a few minutes later, the "star" disappeared from view. Now the scientists had little doubt - they had just witnessed, for the first time, a powerful near-infrared flare from exactly the direction of the supermassive black hole at the heart of the Milky Way.

"We had been looking for infrared emission from that black hole for more than a decade" recalls another team member, Andreas Eckart of the Cologne University. "We were certain that the black hole must be accreting matter from time to time. As this matter falls towards the surface of the black hole, it gets hotter and hotter and starts emitting infrared radiation".

But no such infrared radiation had been seen until that night at the VLT. This was the wonderful moment of breakthrough. Never before had anybody witnessed the last "scream" from matter in the deadly grip of a black hole, about to pass the point of no return towards an unknown fate.

At the border
A careful analysis of the new observational data, reported in this week's issue of the Nature magazine, has revealed that the infrared emission originates from within a few thousandths of an arcsecond from the position of the black hole (corresponding to a distance of a few light-hours) and that it varies on time scales of minutes.

This proves that the infrared signals must come from just outside the so-called "event horizon" of the black hole, that is the "surface of no return" from which even light cannot escape. The rapid variability seen in all data obtained by the VLT clearly indicates that the region around this horizon must have chaotic properties - very much like those seen in thunderstorms or solar flares.

"Our data give us unprecedented information about what happens just outside the event horizon and let us test the predictions of General Relativity" explains Daniel Rouan, a team member from Paris-Meudon Observatory. "The most striking result is an apparent 17-minute periodicity in the light curves of two of the detected flares. If this periodicity is caused by the motion of gas orbiting the black hole, the inevitable conclusion is that the black hole must be rotating rapidly".

Reinhard Genzel is very pleased: "This is a major breakthrough. We know from theory that a black hole can only have mass, spin and electrical charge. Last year we were able to unambiguously prove the existence and determine the mass of the Galactic Centre black hole. If our assumption is correct that the periodicity is the fundamental orbital time of the accreting gas, we now have also measured its spin for the first time. And that turns out to be about half of the maximum spin that General Relativity allows".

He adds: "Now the era of observational black hole physics has truly begun!"