At 9.10 am on the first day of 2024, the Indian Space Research Organisation will launch XPoSat, short for ‘X-ray Polarimeter Satellite’. Once launched, the satellite will study X-rays emitted by phenomena in space and their polarisation from earth-orbit.
X-rays are electromagnetic radiation whose wavelength is 0.01-10 nanometres (where 1 nanometre is one-billionth of a metre), corresponding to energies of 100-100,000 electron-volt (eV). Electromagnetic radiation is characterised by an electric field and a magnetic field vibrating perpendicular to each other. The polarisation of electromagnetic radiation refers to the orientation of these two fields as the radiation moves through space.
X-rays can be polarised when they get scattered. For example, when an X-ray travelling through space encounters an atom, the electric field of the X-ray can energise an electron, which will then emit a photon. Since X-rays are also photons, the new photon will give the impression that an X-ray photon has been scattered. Polarised X-rays are also produced when the path of a fast-moving charged particle is bent by a magnetic field.
Studying these X-rays can reveal which way the magnetic field is pointing, and tracking how these X-rays evolve in time can reveal many things about the body producing such fields, like a pulsar.
The XPoSat satellite has two payloads to make these measurements. POLIX will study X-rays with energy 8-30 keV emitted by around 40 astronomical sources in five years. XSPECT will study X-rays with energy 0.8-15 keV and track changes in continuous X-ray emissions.
