  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.
