The radiation that streams across space from pulsars, black holes and other astronomical objects carries information in its intensity, frequency composition, spatial distribution and polarization. But the polarization of X-rays is particularly difficult to measure - attempts to study it have been hampered by poor sensitivity and have only succeeded for a few very intense sources. Now a team of astronomers led by Enrico Costa at the Istituto de Astrofisica del CNR in Rome has devised a probe to measure the polarization of X-ray sources a hundred times dimmer than those previously observed (E Costa et al 2001 Nature 411 662).
Astronomical objects such as the active galactic nuclei radiate linearly polarized radiation. Spinning bodies, such as pulsars and black holes, emit synchrotron radiation that is elliptically polarized – the vector of polarization rotates about a point, tracing out an ellipse. Astronomers believe that understanding the behaviour of this polarization at X-ray wavelengths will provide crucial clues about the internal structure of these astrophysical entities and also give insights into how matter behaves in extremely intense magnetic and gravitational fields.
The new device is based on the photoelectric effect. X-rays enter a chamber within the detector – filled with neon and dimethyl ether gas – and eject electrons from these gas atoms. The direction of flow of the electrons coincides with the direction of the electric field of the incoming X-ray photons, and therefore reveals the polarization of the incoming X-rays. The trickle of photoelectrons is boosted by an electron multiplier in the ‘micro-pattern gas chamber’, and the subsequent energy loss of the photoelectrons is gauged, providing extra information about the initial kinetic energy of the electrons.
So far, Costa and colleagues have only operated their device on Earth. But they believe that – mounted on a space-based telescope – their instrument will lay the foundations for the new field of practical X-ray polarimetry.
