Wednesday, 23 July 2014
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Physicists seek to determine the origin of cosmic X-rays

UK researchers are hoping to determine the origin of cosmic X-rays using new satellite technology scheduled for launch by NASA today.

Physicists from Durham University want to use the first space telescope that can focus high-energy X-rays using specially coated mirrors — part of the $170m (£110m) NuSTAR satellite — to confirm whether background radiation in space comes from black holes.

These X-rays aren’t detectable on Earth and were first discovered in space in 1962, but scientists have so far been unable to focus the radiation sufficiently to pinpoint its origin.

‘Black holes can pull in gas off nearby stars, and this gas gets very hot and emits strong X-rays,’ Prof David Alexander, the research project’s co-ordinator at Durham, told The Engineer.

‘We’ve known from telescopes that look at lower-energy X-rays that this is likely to be the case, but you don’t know [for sure] until you’ve looked [at higher energy levels] and you don’t know what the exact properties of the objects will be.’

High-energy X-rays can penetrate the gas and dust surrounding black holes so the scientists should be able to track them to their source, but this high penetration rate also makes them very difficult to reflect with focusing mirrors.

Conventional X-ray mirrors are made from high-density materials and are placed almost parallel to the beam’s direction of travel so the X-ray just grazes the surface, increasing the amount of reflection.

But these materials are not so good for reflecting high-energy X-rays, so NuSTAR will use mirrors that alternate the high-density platinum and tungsten with low-density silicon and siliconcarbite in hundreds of layers to form a ‘depth-graded multilayer’.

As the waves hit the different material layers, they produce slightly different patterns that interfere with one another and enhance the beam as it is reflected.

‘When previously we’d have an out-of-focus image, we’ll see things clearly for the first time at these energies,’ said Alexander.

‘We’ll be able to pinpoint where this X-ray emission is coming from and at the same time we can go about 100 times deeper than we’ve been before in the sense that we can see fainter objects.’

Several teams of academics from across the US and Europe helped to develop the telescope, which consists of 130 concentric mirror shells made by depositing the multilayer coating on a flexible glass substrate heated to give it the desired curvature.

The telescope is due to be deployed using a mast that folds out from the satellite once it is in orbit, extending up to 10m in order to position the optics at the right distance from the X-ray detectors to achieve the desired focus.

After a one-month calibration period, the satellite will collect data for an initial period of two years. But Alexander said he hoped the team would begin to make discoveries after just one month of data gathering.


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