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).

AstroSat is the first dedicated space observatory (2015) aimed at studying celestial sources in X-ray, optical, and UV spectral bands simultaneously.

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).

X-rays X-ray is a form of electromagnetic radiation with higher energy, high frequency, and shorter wavelength than visible light. It can pass through most objects, including the body, and produce images of internal structure.

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).