In astronomy there are basically four kinds of observations to extract the information carried by electromagnetic radiation: photometry, imaging, spectroscopy and polarimetry. By optimal exploitation of the first three techniques, X-ray astronomy has been able to unveil the violent world of compact high energy sources. Here we report on a new instrument that brings high efficiency also to X-ray polarimetry, the last unexplored field of X-ray astronomy. It will then be possible to resolve the internal structures of compact sources which otherwise would remain inaccessible, even to X-ray interferometry1. Polarimetry could provide a direct, visual picture of the state of matter under extreme magnetic and gravitational fields by measuring the radiation polarized through interaction with the highly asymmetric matter distribution (accretion disk) and with the magnetic field. The new instrument derives the polarization information from the track of the photoelectrons imaged by a finely subdivided gas detector. Its great improvement of sensitivity (at least two orders of magnitude) will allow direct exploration of the most dramatic objects of the X-ray sky.
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