Artist's concept of XMM in orbit. Photo: ESA

XMM-Newton is blessed with good luck! During the current calibration campaign of its science instruments ESA's new X-ray observatory has chanced on a sudden and dramatic alteration in a binary star system, whose properties had not changed for thirty years.

Located in the Large Magellanic Cloud, the luminous X-ray binary system LMC X-3, is probably - like the other well known candidate Cygnus X-1 - the home of a black hole. The system is constituted of a "B"-type star, similar to our Sun, but much hotter and more massive. Its unseen companion is likely to be a black hole, which cannot be observed directly because nothing, not even light, can escape from it.

The effect of a black hole however can be seen on any object unfortunate enough to be close-by. In the case of LMC X-3, the gravitational field of the black hole has been seen to be pulling and pushing its companion star in their 1.7 day orbit.

A black hole's intense gravitational field also results in hot gases being sucked from the atmosphere of the companion star into a swirling accretion disk, before disappearing in the event horizon or "never-never land" of the black hole. As the gas falls into this abyss, it becomes very hot and emits the X-rays that XMM-Newton can detect.

Data from the EPIC cameras display on the horizontal axis the energy (or color) and vertically the intensity of the X-rays from LMC X-3. The black curve shows the response when the X-ray binary was in its normal state. The red curve is obtained in the new lower intensity observation, almost 100 times fainter. Astronomers are wondering why the shapes of the two curves are different. It could indicate that the temperature of the accretion disk has changed as well as dimmed. Photo: K. Mason, MSSL, UK

XMM-Newton was observing LMC X-3 as a perfect calibration target - bright, stable and well documented - to verify what is called the "point spread function" of its instruments, a measure of the image shape a distant point source of X-rays will produce. The X-ray binary's properties seen by XMM-Newton could easily be compared with previous observations.

The calibration campaign was proceeding quite smoothly when on 19 April the properties of the source suddenly changed and its intensity practically dropped off the scale - a hundred times weaker! Such a pronounced change had never been seen before in the 30 or so years that LMC X-3 has been observed.

The event was also seen by the US Rossi X-ray Timing Explorer satellite which is constantly monitoring the X-ray sky. It was reported in an International Astronomical Union telegram, the alert service carrying the hottest news for astronomers. But XMM-Newton was lucky enough to be also observing at the right place and the right time. Data from its X-ray instruments, and the optical and UV data from its Optical Monitor is now being carefully studied.

For the moment, astronomers cannot explain this rare phenomenon. Models advanced in the past have featured changes in the accretion disks created by the intense gravitational field of black holes. But these explanations remain sketchy, and so the full story of LMC X-3 remains to be told.