ISRO's AstroSat accomplishes X-ray polarisation for the first time

Image courtesy: ISRO

New Delhi: Marking a new era in the country's space program, the Indian Space Research Organisation (ISRO) launched its first dedicated multi-wavelength space observatory – AstroSat – in Spetember 2015.

Now, two years later, the observatory has accomplished the extremely strenuous job of X-ray polarisation.

Putting up a strong challenge to prevailing theories of high energy radiation from pulsars, AstroSat accomplished the 18-month-long task of measuring X-ray polarisation of Crab pulsar in the Taurus constellation for the first time.

"AstroSat accomplished the difficult task of measuring X-ray polarisation of Crab pulsar in the Taurus constellation during 18-months of its study for the first time," ISRO said in a statement here on Monday.

In a paper published in the reputed journal 'Nature Astronomy', a team of Indian astronomers has documented the results of the study and measured the variations of polarisation as this highly-magnetised neutron star spins around 30 times every second.

A pulsar is a rotating neutron star or white dwarf, which emits electromagnetic radiation. Neutron stars and black holes are examples of such objects.

Taurus is the second astrological sign in the present zodiac. It spans the 30-60th degree of the zodiac.

The landmark measurement by the satellite's CZT (Cadmium-Zinc-Telluride) imaging instrument challenges theories of high energy X-ray emission from pulsars.

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.

"Indian scientists using data from the CZT Imager 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," the scientific paper pointed out.

The 1,513 kg space observatory built at a cost of Rs 180 crore has five scientific instruments to study celestial objects in the universe for the first time at optical, ultraviolet and x-ray wavebands simultaneously. The instruments are a soft x-ray telescope, an ultraviolet imaging telescope, an imager and a scanning sky monitor.

A pulsar is a celestial body, believed to be made up on 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.

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 the missing information about the elusive properties of X-rays known as polarisation.

"The idea of using CZT Imager for X-ray polarisation measurement has been around for a while, but this is the first time the idea was tested on ground before launch and then employed in space. This makes our measurements credible," said ISRO's Ahmedabad-Physical Research Laboratory (PRL) Professor Santosh Vadawale in the paper.

Another feature of the measurements is the study of polarisation properties at different rotation phases of the pulsar, which has not been done so far, reiterated Vadawale, lead author of the paper and part of the CZT Imager team.

According to Mumbai-based Tata Institute of Fundamental Research (TIFR) Professor and Imager's principal investigator A.R. Rao, the team had to observe the Crab pulsar many times and combine data of months given its small rotation period.

"To get the micro-second accuracy for combining the data, the AstroSat team sought help from one of the world's best radio telescopes - the Indian Giant Meter-wave Radio Telescope (GMRT), at Khodad near Pune.

Professor Bhal Chandra Joshi and a team from NCRA, Pune, monitored the radio pulsations from Crab with GMRT and Ooty radio telescope, corrected anomalies and provided accurate ephemeris to the imager.

(With Agency inputs)