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NASA funds balloon-borne
X-ray telescope

by Chloe Partridge
Posted: 11 January 2012

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A new X-ray telescope developed by an international team of scientists will float within the Earth’s atmosphere on a one-day mission to calculate how fast black holes spin.

Dr Henric Krawczynski, professor of physics at Washington University, has received NASA funding to explore some of the most exciting X-ray sources in space, namely black holes. The balloon-borne telescope X-Calibur, which will be flown in spring 2013 or autumn 2014, will float 40 kilometres above the Earth in the stratosphere. Here it will detect "hard" X-rays with energies between 20 - 60 kilo electron volts (keV), which have a higher penetrating energy than “soft” X-rays, which have energies less than 12 keV. The GEMS satellite (Gravity and Extreme Magnetism) lead by Dr Jean Swank of the Goddard Space Flight Center, with which Krawczynski is also working, will be flown at roughly the same time as X-Calibur and will be sensitive to soft X-rays.

Artist’s impression of the binary system Cygnus X-1 which will be one of X-Calibur’s first targets. The image shows how Cygnus X-1 slowly draws in gas and material from its companion star (left) forming a flattened accretion disc. As the material spirals inwards towards the centre of the black hole temperatures rise, resulting in the emission of X-rays. Image: (ESA. Illustration by Martin Kornmesser, ESA/ECF).

X-Calibur will measure the direction and degree at which incoming X-rays are polarised, a property that describes the orientation of the X-rays' oscillations. When light is unpolarised, the direction of vibration of the wave is random, but when it is polarised, it vibrates in a certain direction. By studying the poalarisation of X-ray sources like black holes – which are thought to reside at the hearts of most galaxies – astronomers will be able to determine the rate at which they are spinning.

A 1.6 ton gondola developed by the Goddard Space Flight Centre (GFSC) will hang from the balloon and – among other instruments – will carry a special X-ray mirror developed by Hideyo Kunieda of Nagoya University comprising 256 nested mirrors that act as a lens, focusing the X-rays onto a scintillator rod. The rod, also developed by GSFC, then scatters the X-ray photons into a ring of detectors which surround it.

“If you can measure the directions in which the photons are scattered, you can infer the polarisation direction of the X-rays,” explains Krawczynski. “But designing an instrument to detect polarisation is difficult; we need a lot of photons to measure it accurately. Whereas physicists can measure the energy or direction of a single photon, we need as many as 10,000 photons to detect a five percent polarisation signal with high confidence.”

To minimise systematic measurement errors, the telescope will be suspended in the gondola by a single high-pressure ball joint, whilst slowly spinning. This allows the optical axis to remain firmly pointed in the same direction, independent of its surroundings.

X-Calibur will be installed on a gondola (above) suspended from a balloon. Image: NASA/Goddard Space Flight Center.

X-Calibur’s main target is the well-known X-ray source Cygnus X-1, which is widely believed to be a black hole. By studying the X-rays emitted from Cygnus X-1, along with its plasma outflow, the X-Calibur mission can calculate the rate at which the black hole spins. “This is also one of the things GEMS will be able to measure” says Krawczynski. X-rays which are emitted from the outer edge of Cygnus X-1’s accretion disc become polarised parallel to the plane of the disc, return to the disc where they are then scattered towards the observer. This is due to curving of space-time caused by black holes such a Cygnus X-1. “The net effect,” continues Krawczynski, “is that we will see a 90-degree polarisation swing produced by the gravity of the black hole.”

It is this 90-degree polarisation swing which determines how fast the black hole is spinning. If a black hole is rotating at high speeds then the accretion disc will shrink as a result. This means that energy at which the polarisation swing is observed will be lower.

X-Calibur will also study the Crab Nebula pulsar, another black hole (GRS 1915+105), an accreting neutron star (Hercules X-1), and one supermassive extragalactic black hole known as Markarian 421. The experiment also holds high hopes of testing the theory of general relativity near a black hole. The one-day mission, though, is “just the first flight” says Krawczynki. “Longer follow-up flights are envisioned for the years 2014-2019. We’re also proposing a satellite version of the experiment.”

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