AstroSat’s take on Crab nebula baffles astrophysicists


The Cadmium-Zinc-Telluride Imager (CZTI), an instrument to observe and image hard X-rays on board the Indian space observatory AstroSat, has consistently been making important observations since AstroSat’s launch in 2015. The latest discovery, published in Nature Astronomy, is a polarization analysis of the Crab nebula pulsar that has completely baffled astrophysicists studying pulsars.


  • In this work, observations of the Crab pulsar made by CZTI have been analysed in the so-called phase-resolved X-ray polarimetry – a measure of the polarization of X-ray beams emanating from it.
  • The experiment determines the magnitude and orientation of the polarization of the hard X-ray beams. This is the most sensitive and precise measurement of this variable until now.
  • An analysis revealed that the values are contrary to what is predicted by all existing theories of pulsars.
  • Stars that have masses beyond a critical value of about 1.4 times the mass of our Sun will in the course of their lifetime explode to form a supernova.
  • Even as some matter escapes from the explosion to create a glow, the remnant at the centre shrinks to become a black hole or a neutron star.
  • The Crab nebula, in the Taurus constellation, is one such supernova remnant that has become a type of neutron star known as a pulsar.
  • Known as the Crab pulsar, this emits electromagnetic radiation in a beam and also spins rapidly so that distant observers see the beam as a pulsating spot of light, justifying the name “pulsar.”

Million cycles:

  • “The Crab pulsar pulses once every 0.33 seconds and the data acquired by CZTI after observing millions of such 0.33-second cycles over a period of a year and half were analysed to get this result.
  • The pulses of radiation from the Crab pulsar show two peaked shapes coinciding with its north and south poles. In between these two high regions is a low-intensity, zero point called the off-pulse region.
  • CZTI detected non-vanishing polarization having a definite varying structure in the off-pulse region where no variation was expected. “Since in the off-pulse region is dominated by radiation from the nebula [the cloud-like matter spreading away from the centre], the polarisation is expected to remain a constant here.
  • But it certainly swings [varies with a definite shape].
  • Existing theories predict that there should not be such a variation of the polarisation.
  • However, since the experiment has been repeated several times and the signs persist, it has forced theorists to rethink their theories of pulsars.
  • The extremely sensitive measurement was possible mainly because of the way the instrument was built and systemic errors were understood.


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