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  • 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-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).
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