A star spectacularly exploded about 7,500 years ago. Situated at about 6,500 light years away, the dazzling light from the explosion reached the earth during 1054 CE. The luminosity of the explosion was brighter than million suns, and was hence even visible in day light to the wonderment Chinese astronomers. They were amazed at the appearance of a new bright star in the sky and noted it in their record. This is the earliest record of the crab nebula supernova explosion.

Many years later, with the advent of telescope, astronomers found a nebula, a cloud like structure. It became clear that it is the remnant of an exploded star. It was in the 1960s, astronomers could detect the rapidly rotating neutron star- a pulsar- inside a gas envelop. The massive core of the exploded star had collapsed into the neutron star, one of the most dense objects in the universe.

Now Indian astronomers, wielding powerful Cadmium–Zinc–Telluride Imager (CZTI) on-board the Indian astronomy satellite, AstroSat, have obtained the ‘most precise hard X-ray polarization measurements of the Crab pulsar so far’. The discovery has been announced on Monday in journal Nature Astronomy. Since the launch of AstroSat in September 2015, the Crab nebula has been extensively observed on 21 different occasions during its first 18 months of operation.

Some of the neutron stars are highly magnetised and rotate at rapid pace. They emit a beam of light and other electromagnetic waves, just like a lighthouse, in a particular plane. Due to pulsed appearance of emission, these neutron stars are called as pulsars. When the beam is pointed towards the earth, astronomers can detect it.

“Crab nebula hosts a Crab pulsar, which is a typical example of a young, rapidly spinning, strongly magnetized neutron star that generates broadband electromagnetic radiation by accelerating charged particles to near light speeds in its magnetosphere,” Prof Santosh V. Vadawale of Physical Research Laboratory, Ahmedabad, lead author of the study, told India Science Wire.

With just a size of 28–30 km in diameter, the Crab pulsar contains 1.4 solar masses and rotates thirty times every second emitting a pulse of radiation in almost all wavelengths every 33 milliseconds.

Because they are so tiny it is not practical to look at pulsars through telescope to study its shape and dynamics. The spectra of the pulsar can be used to calculate physical dimension including the mass density of neutron star, while time variability in the pulses give the absolute physical dimension. Just as you cannot solve two variables with just one equation, if one had to pinpoint the dynamics of neutron stars we need additional information.

Degree and direction of x-ray polarisation help in understanding neutron stars better. The polarisation of x-rays can occur when charged particles move in strong magnetic fields of pulsar. The x-rays scattered from surrounding materials also give raise to polarisation. “Investigating the polarisation of emitted x-rays is a good diagnostic to study the location and fundamental mechanisms behind emission processes,” explained Prof A. R. Rao of TIFR.

The reception of emission when the beam is turned towards the earth is natural. However, this research has found “polarization is varying the most in the ‘off-pulse’ mode, when the beam

is turned away from the earth, 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” says Prof Dipankar Bhattacharya of Inter-University Centre for Astronomy and Astrophysics (IUCAA).

Hard X-ray emissions observed by CZTI come from all over the nebula, whereas the optical band includes only those from pulsar. “When we compare data collected in optical wavelengths, we find variation in off-pulse emission in hard X-ray. This means these hard x-rays originated in the pulsar itself and not from the surrounding materials,” said Prof Vadawale. Further they also detected difference in polarisation in optical and x-ray band just trailing a pulse peak. What this implies is yet to be ascertained.

Using available data, astrophysicists developed many models to explain pulsars. The results from this study rules out two of the most popular models and raises questions on the rest, forcing the astrophysicists to go back to their drawing boards. Incipient attempts to study x-ray polarisation from pulsars commenced in early 1970s, the work came to standstill in 1978 due to lack of sensitise instrumentation. AstroSAT has restarted this exploration and soon ISRO is going to launch a dedicated satellite mission XpoSat with X-ray Polarimeter (POLIX) aboard to study X-ray polarisation measurement of hard X-ray sources.

The research team included S. V. Vadawale, T. Chattopadhyay, N. P. S. Mithun (all from Physical Research Laboratory, Ahmedabad); A. R. Rao (TIFR, Mumbai); D. Bhattacharya, A. Vibhute, G. C. Dewangan, R. Misra (all from IUCAA, Pune); V. B. Bhalerao (IIT Mumbai); B. Paul (RRI, Bengaluru); A Basu, B. C. Joshi (NCRA Pune) and S. Sreekumar, E. Samuel, P. Priya, P. Vinod (all from VSSC, Thiruvananthapuram) and S. Seetha (ISRO, Bengaluru). (India Science Wire)

Twitter handle: @TVVen

(India Science Wire)