XPoSat Mission

• Context (IE | PIB): ISRO launched PSLV-C58 with XPoSat as the primary payload in lower Earth orbit.
• XPoSat will become the country’s third space-based observatory after the solar mission Aditya-L1 and AstroSat (launched in 2015).

Difference from Earlier Missions

• Earlier, Polarisation of celestial sources was done either in the optical or radio bands.
• For the first time, X-ray polarisation will be measured in the medium energy band (8-30 keV).
• XPoSat will observe two kinds of sources — persistent sources and transient sources (pulsars, active galactic nuclei, magnetars).

Polarisation of X-rays

• As X-ray light passes through a material, the electric part of the electromagnetic wave causes electrons to emit a photon.
• The emission of photons gives the appearance that the original photon has changed direction or has been scattered.
• Cosmic X-rays emitted from magnetars or black holes encounter a variety of materials in the Universe.
• Such interactions emit a photon in a changed direction due to scattering.
• The angular and degree of polarisation measurements are believed to provide clues about the nature of these radiations and the complex process they undergo.
• Study of these polarisations will lead to discoveries about black holes and other cosmic events.

Other X-ray Missions

• NASA’s HX-POL and XL-Calibur have been balloon-based and short-duration experiments.
• Indian AstroSat performed timing and broadband spectroscopy of X-ray sources, but no polarisation studies were performed.
• NASA launched Imaging X-ray Polarimetry Explorer (IXPE) in 2021, within the soft X-ray band (2 to 8 keV energy band).