AstroSat’s take on Crab nebula baffles astrophysicists

Analysis reveals unexplained patterns

November 25, 2017 08:20 pm | Updated 08:22 pm IST

 This NASA-ESA image shows the entire Crab Nebula.

This NASA-ESA image shows the entire Crab Nebula.

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.

Polarimetry

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,” says Varun Bhalerao of Indian Institute of Technology, Bombay, one of the researchers involved.

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],” says Dr Bhalerao.

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, he adds.

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