XPoSat is ISRO's first satellite dedicated to measuring the polarisation of X-rays from black holes, neutron stars and other bright cosmic sources. PSLV-C58 launched it from Sriharikota on 1 January 2024.
Telescopes usually record how bright an X-ray source is and at what energies. XPoSat adds two more clues: the degree and the angle of polarisation of the X-rays. These help astronomers test competing ideas about how black holes, neutron stars, pulsar wind nebulae and active galactic nuclei produce their radiation.
It is the world's second mission dedicated to X-ray polarimetry, after NASA's IXPE.
XPoSat is built on a modified IMS-2 bus, with mainframe systems based on India's IRS satellites. It weighs 469 kg and has a planned life of about five years.
PSLV-C58 lifted off at 09:10 IST on 1 January 2024 and placed XPoSat in an eastward, low-inclination orbit. The public catalogue lists it at about 624–638 km, inclined 6° to the equator. After the release, the rocket's fourth stage was restarted and turned into the POEM-3 experiment platform.
POLIX was expected to study about 40 bright sources over the mission. It is the first instrument in the medium X-ray band built only for polarimetry.