Balloon-borne telescope will shed new light on black holes

A balloon-borne telescope will shed new light on black holes by looking at the polarisation of X-rays from outside the solar system.

Researchers from Washington University in St Louis, Missouri, have received funding from NASA to launch the X-Calibur telescope. It will float at an altitude of 40km, studying black holes both in and outside of our galaxy.

Astronomers have previously only studied X-ray polarisation from one source outside the solar system, the Crab Nebula. X-Calibur and another mission called GEMS will study new sources and provide a way of gathering data not previously available.

‘Whenever you look at the sky at a different wavelength you see something completely different,’ said research leader Prof Henric Krawczynski.

‘[X-Calibur’s] main claims to fame are two new observables: the polarisation degree and direction of X-rays, which provide information about cosmic sources that is not available in any other way.’

Once airborne, the telescope will spend a day studying two black holes in our galaxy, an extragalactic black hole, an accreting neutron star, the Crab nebula, and other targets yet to be chosen. It is scheduled for launch in spring 2013 or autumn 2014.

‘The most exciting targets for the telescope are the black holes and their plasma outflows,’ said Krawczynski. ‘One of the things GEMS and X-Calibur will be able to measure is how fast the black holes are spinning.’ It may also be able to test the theory of general relativity near a black hole.

X-Calibur will sit suspended in a gondola to maintain its alignment

X-Calibur is sensitive to ’hard’ X-rays with energies of 20,000 to 60,000 electron volts, whereas the satellite-borne GEMS detects ’soft’ X-rays with energies of 2,000 to 10,000 electron volts. By comparison, visible light has two to three electron volts.

‘Designing an instrument to detect polarisation is difficult because we need a lot of photons to measure it accurately,’ said Krawczynski

‘Whereas physicists can measure the energy or direction of a single photon, they need as many as 10,000 photons to detect a five per cent polarisation signal with high confidence.’

X-Calibur works by using a scintillator rod to scatter X-rays into rings of detectors surrounding it. By measuring the direction in which the photons are scattered, the researchers can infer the polarisation direction of the X-rays.

The assembled instrument will be flown in a 1,600kg gondola developed by the Goddard Space Flight Center. The telescope is suspended in the gondola by a single high-pressure ball joint, slowly spinning to minimise systematic measurement errors.

This enables it to act like the rotor in a gimballed gyroscope so that, no matter how its surroundings move, its optical axis remains firmly pointed in the same direction. The researchers need to focus the telescope with an accuracy of a 60th of a degree.

Also aboard the gondola is an X-ray mirror developed by Hideyo Kunieda from Nagoya University in Japan. This consists of 256 nested cylindrical mirrors that act as a lens, focusing the X-rays onto the tiny scintillator pin at a distance of 8m.