The main instrument on XPoSat, India's first X-ray polarimetry mission. It measures how X-rays from bright cosmic sources are polarised.
Light waves can vibrate in a preferred direction; this is called polarisation. The degree and angle of polarisation of X-rays carry clues about magnetic fields and the shape of the region around black holes and neutron stars. POLIX measures both in the 8–30 keV range. ISRO describes it as the first payload dedicated to polarimetry in this medium X-ray band.
X-rays enter through a collimator that limits the view to a 3° × 3° patch, so usually only one bright source is in view. They strike a block of light material (the scatterer). Polarised X-rays scatter more strongly in some directions than others, and four X-ray detectors around the scatterer measure that pattern.
The Raman Research Institute in Bengaluru developed POLIX with ISRO's U R Rao Satellite Centre. PSLV-C58 launched XPoSat on 1 January 2024. Over a planned life of about five years, POLIX is expected to observe about 40 bright sources.