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Astronomers, including Sera Markoff and Dave Russell of the University of Amsterdam (UvA), have observed the flaring jet (stream) from a black hole using NASA's infrared telescope WISE.

Astronomers, including Sera Markoff and Dave Russell of the University of Amsterdam (UvA), have observed the flaring jet (stream) from a black hole using NASA's infrared telescope WISE. Sudden, random flashes lead to the jet stream becoming three times as bright in a matter of hours. The results were published in Astrophysical Journal Letters on Tuesday, 20 September 2011.

The astronomers were able to use WISE’s infrared camera to zoom into the inner regions of the base of the jet for the first time. ‘As a result, we were able to measure the physical properties in unprecedented detail, as well as fluctuations in the jet activity on very small times cales,’ reported lead author Poshak Gandhi from the Japan Aerospace Exploration Agency.

Black hole GX 339-4 has been observed previously. It is over 20,000 light years away near the centre of the Milky Way and has a mass at least six times greater than that of the Sun. Black holes are the remnants of very heavy stars that collapse under their own gravity at the end of their lives. GX 339-4, like other black holes, is a dense collection of matter, with a gravitational field so great that not even light can escape. There is also a companion star that feeds it material. Most of this material is sucked into the black hole, but some of it as blasted away as a jet stream flowing at almost the speed of light.

Right time, right place

Due to the X-ray, gamma and radio emissions viewable from the jet stream, scientists have learned a lot about the accretion disks (the material that feeds the black hole) and the jets themselves. However, it was difficult to investigate the brightest parts of the jets (at the base) before the infrared instrument WISE was put into operation.

‘To see bright flaring activity from a black hole, you need to be looking at the right place at the right time,’ says Peter Eisenhardt, the project scientist for WISE at NASA’s Jet Propulsion Laboratory in Pasadena, California. ‘WISE snapped sensitive infrared pictures every 11 seconds for a year, covering the whole sky, allowing it to catch this rare event.’

The findings surprised astronomers. They saw large and irregular changes in the activity of the jet, ranging from 11 seconds to a few hours. Never before has this been recorded with such precision. The observations show that the size of the jet’s base varies, with considerable changes by as large as a factor of 10 or more.

The astronomers have made the most accurate measurements ever of the magnetic field of a black hole, which is 30,000 times stronger than Earth's.

The scientists are now going to search for more flickering black holes using the WISE data they have collected. ‘The power of jets from black holes is a complicated process," says UvA astronomer Sera Markoff. ‘It involves strong gravitational magnetohydrodynamica (the motion of an electrically conducting gas in a magnetic field). Our observations provide important new models that our models need to fulfill, such as the geometry of the jet, the strength of the magnetic field and the density at the base of the jet. This brings us closer to understanding this kind of exotic phenomena,’, says Markoff.