PoGOLite and the cosmic X-rays' oscillations

A prototype of PoGOLite that was tested in Japan recently. Credit: Mark Pearce.

The researchers behind the balloon-borne instrument PoGOLite have in a preparatory study investigated what results they can expect on the polarization of cosmic X-rays.

Only a few measurements of polarization have been made on cosmic X-rays. A new instrument, PoGOLite, is being constructed to remedy this. Already before construction is complete, a recent study has shown that it can provide us with new information about X-ray polarization. Researchers from the High-energy Astrophysics group at Stockholm Observatory and the Particle and Astrophysics group at KTH are part of the study.

Electromagnetic waves (such as light) can oscillate in different directions. When the radiation oscillates preferentially in one direction, it is said to be polarized. PoGOLite is an instrument designed to measure polarization from cosmic X-ray sources. This group includes some of the most exciting objects in the Universe: neutron stars, active galactic nuclei, and accreting black holes. For many of these sources, PoGOLite will give us the first ever measurements of polarization. Unlike previous measurements made, PoGOLite will be able to measure polarization in a wide energy band. Besides measuring the total polarization, this means that we will be able to see how it varies with photon energy.

The study now being published in an article by Magnus Axelsson et al. has investigated how well PoGOLite can measure polarization changes in energy. Since the instrument has not yet been completed, simulations have been used. Previous observations have shown how strong the radiation in PoGOLite's energy range is from different sources. By adding assumptions on how strong the polarization is at different energies, a realistic model of a given source is created. A computer model of the instrument is then used to simulate how the instrument reacts to the source model. This gives a good approximation to what the real instrument will show. The results indicate that PoGOLite will be able to detect changes in polarization unless they are quite small.

-Theoretical models for various cosmic X-ray sources indicate that the polarization does vary with energy, says Magnus Axelsson, Ph.D. student at Stockholm Observatory. But since no such measurements have been made, we don't know what really happens. So the result that PoGOLite will be able to see whether the polarization really changes between different energies is quite exciting. It will however be a while before the real measurements are made, since the first flight with PoGOLite will earliest be made in 2009.

The Earth's atmosphere shields us from cosmic X-rays, so the instrument must be sent to high altitudes or be put on a satellite to observe this radiation. PoGOLite will be flown on a balloon at an altitude of 40 km. Only a few places in the world can launch these balloons, among them the space center Esrange in Kiruna. Both short (more than 6 hours) and long (about 5 days) duration flights are planned.

PoGOLite is currently being built, and a prototype has been tested in the laboratory. The project is a collaboration between research groups in Japan, Sweden, the USA, and France.