X-ray polarisation of Crab pulsar: ASTROSAT, India’s multi-wavelength space telescope, measures star in Taurus constellation

India’s multi-wavelength space telescope, ASTROSAT has successfully completed the extremely difficult task of measuring X-ray polarisation. This accomplishment has put up a strong challenge prevailing the theories of high energy radiation from pulsars. A pulsar is a celestial body, believed to be made up of neutrons, and rotates 30 times per second. Even though they are comparatively tiny in size, they have mass more than that of the entire Sun, and hence are known as compact objects.

According to reports, published in the reputed journal 'Nature Astronomy', a team of Indian astronomers has documented the results of their 18- month study of the Crab Pulsar in the Taurus constellation and measured the variations of polarisation as this highly-magnetised neutron star spins around 30 times every second.

Scientists and astronomers, who participated in the project hail from the country's top scientific institutes -- the Tata Institute of Fundamental Research (TIFR), Mumbai; the Vikram Sarabhai Space Centre, Thiruvananthapuram; ISRO Satellite Centre (ISAC), Bengaluru; The Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune; and Physical Research Laboratory (PRL), Ahmedabad.

Having an extremely large mass condensed in a very small volume, these objects possess extremely strong gravitational and magnetic fields, both approximately trillion times that on the Earth.

"They are known to be sources of intense X-ray radiation, electromagnetic waves similar to light but having ten to hundred thousand times higher energy, which carries vital clues to understand them as well as the physical processes responsible for the radiation," the IUCAA said in a statement.

Astronomers have extensively studied various properties of this radiation to piece together a complete picture of compact objects and their immediate surroundings.

However, the puzzle often remains incomplete due to the missing information about the elusive properties of X-rays known as polarisation.

X-ray polarisation measurement is so difficult that so far the only reliable measurement obtained worldwide is for the pulsar in the Crab Nebula -- the ghostly remains of a massive stellar explosion known as a supernova, observed in 1054 AD.

The scientists using data from the CZT (Cadmium Zinc Telluride) imager instrument of the Astrosat satellite, launched in September 2015, have performed the most sensitive measurement of X-ray polarisation of the Crab Pulsar, the rotating neutron star which is the main energy source of the nebula.

"These measurements have, for the first time, enabled the study of polarisation at different rotation phases of the pulsar," the statement added.

It has been observed that the polarisation is varying the most in the off-pulse duration when no contribution from the pulsar is expected, which poses a serious challenge to most of the current theories of how this object produces X-rays, the statement said.

With PTI Inputs