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Black hole 'bullets' may shed light on galaxy jets
by Dr Emma Rigby
for Astronomy Now
Posted: 12 January 2012

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A nearby black hole has been caught in the act of launching high speed 'bullets' of gas into space thanks to observations from NASA's Rossi X-ray Timing Explorer (RXTE) satellite and the American National Science Foundation's Very Long Baseline Array (VLBA). The results, presented this week at the American Astronomical Society meeting in Austin, Texas, could also hold clues to the behaviour of powerful distant galaxies.

"Like a referee at a sports game, we essentially rewound the footage on the bullets' progress, pinpointing when they were launched," said researcher Gregory Sivakoff of the University of Alberta in Canada. "With the unique capabilities of RXTE and the VLBA, we can associate their ejection with changes that likely signalled the start of the process."

This animation illustrates what astronomers think happened in the black hole binary system H1743-322 in June 2009. An X-ray hot spot in the accretion disc surrounding the black hole produces increasingly rapidly varying signals as it spirals inwards, culminating in the eventual ejection of two high speed 'bullets' of gas along the jets. Credit: NASA Goddard's Scientific Visualization Studio.

The black hole in question, which is located 28,000 light-years away toward the constellation Scorpius, has a close binary companion – a normal star – from which it steals a continuous stream of matter, surrounding itself with a massive disc of accreted gas, millions of miles across. As the material swirls inward it is compressed and heated to such a high temperature that it emits X-rays. However, it can also be ejected out of the disc, and away from the black hole, as dual, oppositely directed, jets. Generally the jet particles flow steadily, but occasionally something alters and massive blobs of gas are fired out at around a quarter of the speed of light.

In June 2009, with astronomers watching, the binary system underwent such a change. After a period of increased X-ray activity, its radio emission suddenly brightened and a radio-emitting gas bullet was seen, moving outward in the direction of one of the jets. It was followed a day later by the detection of a second, moving in the opposite direction. On closer inspection though it turned out that these bullets were launched several days before the radio flare.

Click here for larger image. Radio imaging from the VLBA (top row), combined with simultaneous x-ray observations by RXTE (middle) reveal the ejection of massive gas 'bullets' by the black hole binary system H1743-322 in June 2009. The observations reveal a link between the this event, the brightening of the radio emission and the disappearance of the X-ray signals. However, instead of happening simultaneously, the X-rays vanished two days earlier than the radio flare that was previously thought to signal the ejection. Image: NRAO and NASA's Goddard Space Flight Center.

"I hate to mix metaphors, but for a moment, let's replace the bullet with the idea of a snowplough at the beginning of its route, and consider the steady jet is snow. At first, there's not much snow on the blade of the plough. It takes time for the snow to build up as the plough moves along its route. This would explain the delay", suggests Sivakoff. "Scientifically speaking the idea is that we're driving a shock wave through the existing material and as the shock wave compresses the material in front of it, it becomes bright enough to see in the radio. While this explains a lot of what we see, there are a few holes in the explanation that we are working on understanding."

Studying systems like this within our own galaxy can help us understand the monster jets launched from the central regions of active galaxies. According to lead researcher James Miller-Jones, from the International Centre for Radio Astronomy Research in Australia, "Black hole jets in binary star systems act as fast-forwarded versions of their galactic-scale cousins, giving us insights into how they work and how their enormous energy output can influence the growth of galaxies and clusters of galaxies".

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