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Magnetic fields whip up a storm around black hole
Posted: 27 March 2011

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The creation of powerful jets of particles blasting out from a black hole has been observed by the European Space Agency’s INTEGRAL (INTErnational Gamma Ray Astrophysics Laboratory) spacecraft, confirming our ideas for what powers these immense beams of radiation.

Bi-polar jets are observed emanating from many black holes, from on the largest scale of the supermassive black hole in the elliptical galaxy M87, to stellar mass black holes like Cygnus X-1 that are consuming gas from unfortunate companion stars. By combining all INTEGRAL’s observations of Cygnus X-1 taken over the past seven years, a team of scientists have identified strong magnetic fields in action just a few hundred kilometres from the black hole’s event horizon.

An artist’s impression of material streaming from the companion star and into a spiral disc around the Cygnus X-1 black hole, where magnetic fields allow some of the particles to escape in jets. Image: ESA.

Cygnus X-1 is what is known as a high mass X-ray binary – a system wherein a massive star is orbiting a dense object, in this case a black hole, which is emitting X-rays. The black hole – the remains of a supernova – has almost nine times the mass of our Sun and its event horizon (which is the region beyond which not even light can escape the black hole’s gravitational pull) is estimated to have a radius of about 26 kilometres. In 2006 Cygnus X-1 was the first stellar mass black hole shown to radiate in even higher energy gamma rays, and now astronomers know why.

INTEGRAL was able to detect polarised gamma rays, indicating they were emitted via a process known as synchrotron radiation, wherein charged particles – electrons and protons – emit radiation of a specific polarisation by spiralling around a magnetic field. The gamma-rays, and hence the magnetic field, were found by INTEGRAL to be coming from a region of the accretion disc of gas around the black hole called the corona.

“It is a very tiny region, extending up to around 400 kilometres away from the black hole,” says Philippe Laurent from CEA Saclay in France, who led the report. “To obtain synchrotron emission in gamma-rays, we need either very energetic electrons or a high magnetic field. For a one Gauss magnetic field, which is comparable to the Sun’s magnetic field strength and which is theoretically plausible for this system, electrons should have energies of teraelectronvolts (TeV).”

The magnetic field is embedded within this disc of ionised gas, which is made of material ripped from the companion star – a 20 solar mass supergiant that will one day explode as a supernova and create a second black hole of its own. The magnetic field seems to be highly structured, allowing some charged particles to be swept up into the jets and escape otherwise certain doom within the black hole. Those that don’t escape are consumed by the black hole a millisecond later.