Indian astronomers obtain the ‘most precise hard X-ray polarization measurements of the Crab pulsar so far’

A star spectacularly exploded about 7500 years ago. Situated at about 6500 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 daylight to the wonderment Chinese astronomers. They were amazed at the appearance of a new bright star in the sky and noted it on their record. This is the earliest record of the crab nebula supernova explosion.

In the Crab nebula's very center lies a pulsar: a neutron star as massive as the Sun. NASA

Many years later, with the advent of the 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 densest 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 6 November 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 a rapid pace. They emit a beam of light and other electromagnetic waves, just like a lighthouse, in a particular plane. Due to the 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,” 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.

AstroSat measurement of the polarization of X-ray emission from the Crab Pulsar. India Science Wire

Because they are so tiny it is not practical to look at pulsars through the 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 a pulsar. The x-rays scattered from surrounding materials also give rise to polarisation. “Investigating the polarisation of emitted x-rays is a good diagnostic to study the location and fundamental mechanisms behind emission processes,” explained A. R. Rao of TIFR.

This composite image of the Crab Nebula was assembled by combining data from five telescopes spanning nearly the entire breadth of the electromagnetic spectrum. NASA

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 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 the 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 Vadawale. Further, they also detected a difference in polarisation in the optical and x-ray band just trailing a pulse peak. What this implies is yet to be ascertained.

At the centre of the Crab Nebula is an extremely dense pulsar which is spewing out a blizzard of high-energy particles. NASA

Using available data, astrophysicists developed many models to explain pulsars. The results from this study rule out two of the most popular models and raise 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 the early 1970s, the work came to standstill in 1978 due to lack of sensitising 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