An efficient photoelectric X-ray polarimeter for the study of black holes and neutron stars

The study of astronomical objects using electromagnetic radiation involves four basic observational approaches: imaging, spectroscopy, photometry (accurate counting of the photons received) and polarimetry (measurement of the polarizations of the observed photons). In contrast to observations at other wavelengths, a lack of sensitivity has prevented X-ray astronomy from making use of polarimetry. Yet such a technique could provide a direct picture of the state of matter in extreme magnetic and gravitational fields(1-6), and has the potential to resolve the internal structures of compact sources that would otherwise remain inaccessible, even to X-ray interferometry(7). In binary pulsars, for example, we could directly 'see' the rotation of the magnetic field and determine if the emission is in the form of a 'fan' or a 'pencil' beam(1,8). Also, observation of the characteristic twisting of the polarization angle in other compact sources would reveal the presence of a black hole(9-12). Here we report the development of an instrument that makes X-ray polarimetry possible. The factor of 100 improvement in sensitivity that we have achieved will allow direct exploration of the most dramatic objects of the X-ray sky.