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Slew Survey Reveals Secrets Of X-Ray Sky


The European Space Agency’s XMM-Newton X-ray observatory has already been a spectacular success in many areas of astronomy - detecting distant clusters of galaxies, the faint afterglow of enigmatic gamma ray bursts and the effects of the collision of the Deep Impact probe with comet Tempel-1.

Now an innovative new approach analyses X-rays detected during the times that the satellite manoeuvres between targets - originally considered to be unusable periods - to reveal some 4,000 intensely brilliant X-ray stars and galaxies.

An initial study has identified these with a host of highly energetic celestial phenomena. These include close binary stars, where matter is being pulled away from one star to explode onto the surface of the companion; and distant quasars, super-luminous galaxies up to 10 billion light years from Earth, which are being slowly consumed by a voracious central black hole.

Preliminary results from this study, known as the XMM-Newton Slew Survey, will be presented today (Tuesday) at the RAS National Astronomy Meeting in Leicester by Dr. Andy Read of Leicester University.

XMM-Newton, with the huge collecting area of its many nested mirrors and the excellent efficiency of its EPIC X-ray camera/detector, is the most sensitive X-ray observatory ever flown. As a result, it is able to record unrivalled numbers of X-rays from the cosmos.

Since its launch in December 1999, XMM-Newton has observed thousands of objects, gazing at a particular X-ray source for hours, before turning or ‘slewing’ away at a great speed to observe its next target. Since September 2001, the XMM-Newton ’shutter’ has been open during these slews, and this has yielded hundreds of extremely long snapshot strips of the X-ray sky.

Though the slews are so quick that XMM-Newton passes over each point in the sky in only 10 seconds (compared to a normal ‘pointed’ observation of a few hours), this 10 second exposure is enough time for XMM-Newton to detect thousands of sources in the sky.

“The area of the sky that is covered is enormous,” said Andy Read. “Over a quarter of the entire sky has already been covered in the 400 or so slews so far performed, and many more slews are continually taking place.

“The entire sky will be covered - even at the present extremely slow rate - over the lifetime of the XMM-Newton mission.

“As such, this XMM-Newton slew survey, even coming as it does, for ‘free’, rivals the best of all previous dedicated all-sky X-ray surveys. For detecting high-energy X-rays, it is certainly the best that there has ever been.

“A wonderful variety of X-ray sources has been seen in the Slew Survey, including black holes, quasars, active galaxies and stars - many of which have been observed for the first time.”

Only a survey such as this, covering such a large area of the sky, is able to observe the rarest of events. The team involved in the survey has, for example, seen a number of X-ray sources undergoing extraordinary changes in brightness, events that are thought to be due to stars being captured whole by massive black holes.

The best example is an otherwise normal galaxy, about 40 million light years away, which has been seen to increase remarkably in X-ray brightness (possibly linked to matter being drawn into a supermassive central black hole).

"This galaxy is far, far brighter than it was when previously observed with older X-ray satellites," said Dr. Read.

Furthermore, the great sensitivity and low ‘noise’ of the EPIC CCD camera is especially suited to detect emission from extended sources that cover a large area of space, and many extremely interesting supernova remnants and massive clusters of galaxies have been observed.

“Multi-slews that are criss-crossing large areas of the sky are now giving us new, large-scale views of the biggest X-ray objects in the sky,” said Andy Read.

Dr. Andy Read | alfa
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