Polapix

When a black hole passes through a cloud of interstellar matter or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. This heating process ionises the atoms and when the atoms reach a few million Kelvin they emit x-rays.  The extreme gravitational field near a spinning black hole not only bends the paths of x-rays, it also alters the directions of their electric fields, thereby polarising them. Measuring the polarisation of such x-rays can reveal the presence of a black hole and provide astronomers with information on its spin.

Polapix is an application of a Gridpix detector to find polarized x-rays originating from black holes. The detector has a 2cm drift gap and will be flushed with a gas that is still to be determined. The purpose is to measure the polarisation direction of polarised x-rays by an interaction of the x-rays with atoms in the gas (the photoelectric effect). The resulting electrons are known to be emitted preferably in the direction of the polarisation. The experiment is to be done in space, in order to avoid any disturbing interaction with the atmosphere.