High energy: X-rays in outer space

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  • Published: Feb 1, 2015
  • Channels: X-ray Spectrometry
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Across the universe

X-ray polarimetry could reveal new information about high-energy astrophysical sources, such as black holes, the bright and active centres of galaxies, compact neutron stars, and gamma-ray bursts. The development of the X-Calibur instrument at the heart of the work. Credit: Journal of Astronomical Instrumentation

X-ray polarimetry could reveal new information about high-energy astrophysical sources, such as black holes, the bright and active centres of galaxies, compact neutron stars, and gamma-ray bursts. The development of the X-Calibur instrument at the heart of the work is detailed in the Journal of Astronomical Instrumentation.

Ever since our ancestors turned their eyes to the night sky, we have gazed with wonder at the pinpricks of light that are the stars and planets, the occasional cometary smudge, the great stellar marvel that is the Milky Way. Today, we gaze at the sky with new eyes, with radio telescopes, ultraviolet and infrared detectors and X-ray instruments that can see way beyond our near neighbours. Our astronomical observations have added to the wonders and now we ask what are the high-energy processes that give rise to the many bizarre phenomena across the Universe. For instance, what is happening in the immediate locale of a black hole? Even the very largest of telescopes cannot answer such questions, we need to recruit instruments that make observations at regions of the electromagnetic spectrum way beyond our vision.

X-ray resolution

Now, the astrophysics research group at Washington University in St. Louis, Missouri, have built an instrument that is capable of measuring the polarization properties of X-rays. When this instrument is put into space it will give us new insights into the whys and wherefores of energy emitted by extreme astronomical objects, the black holes and the neutron stars, formation zones of relativistic plasma jets, for instance.

It is only truly extreme processes in space that generate high-energy particles and emit radiation that has wavelengths in the X-ray region and beyond into the gamma. Unfortunately, given their unimaginable distance these objects subtend a miniscule angle in the sky and are so cannot be resolved spatially with conventional imaging instruments. The solution is to perform indirect measurements of those regions using the polarization properties of the emitted radiation - such as the orientation of the electric field vector of the X-ray photons they emit. These observations are regularly performed at radio and optical wave bands, but sensitive polarization techniques have not yet available for observations at X-ray energies until now.

Washington University's Henric Krawczynski and Matthias Beilicke and their colleagues designed, built and tested an X-ray polarimeter they called X-Calibur to fill this instrumental gap. The instrument may be flown into space as a rocket payload or sent into the far reaches of the upper atmosphere as a scientific balloon payload.

Polarimetric perspective

"Only five years ago, we came up with the first design of the X-Ray polarimeter," Beilicke explains, "two years later we had a working prototype module and now the full instrument is ready to fly on an astrophysics mission." The team, which includes colleagues from the NASA Goddard Space Flight Center in Greenbeld, Maryland, Brookhaven National Laboratory in Upton, New York and Rice University in Houston, Texas, is planning to undertake the first test flight with the instrument carried to an altitude of approximately 40 kilometres in 2016.

The diagram shows the partly assembled heart of the X-Calibur X-ray polarimeter. The X-ray beam enters the scattering slab, which is glowing blue, in the photograph, from the top. X-rays are scattered in the slab with a scattering distribution that depends on the polarization properties of the incoming beam, the distribution is measured with semiconductor X-ray detectors surrounding the scattering slab on all four sides and the digitized signals are sent to readout electronics at the rear of each detector. More details about the project were published in a special Issue of the journal on "scientific balloon capabilities and instrumentation."

"For the next 1.5 years we will be working to implement the detector into an X-ray telescope that will be flown on a 1-day scientific high-altitude balloon flight," Beilicke told SpectroscopyNOW. "During that flight, we hope to measure the polarization properties of the Crab nebula with high detail - beyond other sources. We also are working on a version of the polarimeter that can be flown on a satellite mission."

Related Links

J Astronom Instrument,  2014, 03, 1440008: "Design and Performance of the X-ray Polarimeter X-Calibur"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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