Thursday, March 24, 2011

Integral Spots Matter a Millisecond from Doom

An artist's impression of the Cygnus X-1 black hole system. Gas from a nearby supergiant star spirals down into the black hole but a small fraction is diverted by magnetic fields into jets that shoot back into space. Credits: ESA

ESA’s Integral gamma-ray observatory has spotted extremely hot matter just a millisecond before it plunges into the oblivion of a black hole. But is it really doomed? These unique observations suggest that some of the matter may be making a great escape.

No one would want to be so close to a black hole. Just a few hundred kilometres away from its deadly surface, space is a maelstrom of particles and radiation. Vast storms of particles are falling to their doom at close to the speed of light, raising the temperature to millions of degrees.

Ordinarily, it takes just a millisecond for the particles to cross this final distance but hope may be at hand for a small fraction of them.

Thanks to the new Integral observations, astronomers now know that this chaotic region is threaded by magnetic fields.

This is the first time that magnetic fields have been identified so close to a black hole. Most importantly, Integral shows they are highly structured magnetic fields that are forming an escape tunnel for some of the doomed particles.

The Imager on Board the Integral Satellite (IBIS) was first activated and put through its paces in November 2002. It captured this image during that test phase and shows not only Cygnus X-1 (centre) but also Cygnus X-3 (upper left). High-energy sources are shown with an 'X' followed by a number according to their strength. Cygnus X-3 is the third brightest high-energy emitter in the constellation of Cygnus, the Swan. Instead of a black hole, Cygnus X-3 is thought to be a neutron star (a tiny dead stellar core) pulling its companion star to pieces. Taken on 16 November 2002, the new IBIS observations support this theory. Cygnus X-1 is about 10 000 light years from Earth and one of the brightest high-energy emitters in the sky. It was discovered in 1966 and is thought to be a black hole, ripping its companion star to pieces. The companion star, HDE 226868, is a blue supergiant with a surface temperature of around 31 000 K. It orbits the black hole once every 5.6 days. Credits: ESA. Original image by the Integral IBIS team. Image processing by ESA/ECF.

Philippe Laurent, CEA Saclay, France, and colleagues made the discovery by studying the nearby black hole, Cygnus X-1, which is ripping a companion star to pieces and feeding on its gas.

Their evidence points to the magnetic field being strong enough to tear away particles from the black hole’s gravitational clutches and funnel them outwards, creating jets of matter that shoot into space. The particles in these jets are being drawn into spiral trajectories as they climb the magnetic field to freedom and this is affecting a property of their gamma-ray light known as polarisation.

A gamma ray, like ordinary light, is a kind of wave and the orientation of the wave is known as its polarisation. When a fast particle spirals in a magnetic field it produces a kind of light, known as synchrotron emission, which displays a characteristic pattern of polarisation. It is this polarisation that the team have found in the gamma rays. It was a difficult observation to make.

“We had to use almost every observation Integral has ever made of Cygnus X-1 to make this detection,” says Laurent.

This is an artist’s impression of ESA’s orbiting gamma-ray observatory, Integral.
Credits: ESA

Amassed over seven years, these repeated observations of the black hole now total over five million seconds of observing time, the equivalent of taking a single image with an exposure time of more than two months. Laurent’s team added them all together to create just such an exposure.

“We still do not know exactly how the infalling matter is turned into the jets. There is a big debate among theoreticians; these observations will help them decide,” says Laurent.

Jets around black holes have been seen before by radio telescopes but such observations cannot see the black hole in sufficient detail to know exactly how close to the black hole the jets originate. That makes these new observations invaluable.

"This discovery of polarized emission from a black hole jet is a unique result demonstrating that Integral, which is covering the high-energy band in ESA's wide spectrum of scientific missions, continues to produce key results more than eight years after its launch," says Christoph Winkler, ESA Integral Project Scientist.

Contact for further information

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954

Philippe Laurent
Integral/IBIS Instrument Scientist
IRFU / Service d'Astrophysique, CEA Saclay
Laboratoire APC
Tel: +33 1 69 08 80 66 / +33 1 57 27 60 72

Christoph Winkler
ESA Integral Project Scientist
Tel: +31 71 565 3591

Notes for editors

Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1 by P. Laurent et al. is published online by Science today and will appear in a future issue of the printed journal.