ASTROSAT, India’s multi-wavelength space telescope, has successfully accomplished the extremely difficult task of measuring X-ray polarisation. In a paper published in the journal Nature Astronomy on November 6, 2017, the Astrosat team has documented the results of their 18-month study of the Crab pulsar in the Taurus constellation and measured the variations of polarisation as a highly-magnetised and exotic object that spins about 30 times every second. However, an official release issued from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), stated on Monday that the landmark measurement put up a strong challenge to prevailing theories of high energy X-ray emission from pulsars.
X-ray polarisation measurement has been described as something so difficult that, by 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 supernova, which was observed in 1054 AD.
Indian scientists used the data from the the CZT Imager (CZTI) — an instrument that derives its name from the detectors that make up its heart. The CZTI is made of a semiconductor called “Cadmium Zinc Telluride” and consists of 16,384 tiny pixels, essentially independent detectors, to measure X-rays coming from the celestial objects. The instrument of the AstroSat satellite 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. 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.
Professor Santosh Vadawale, who is the lead author of the paper and member of the CZTI team, said, “The idea of using pixelated CZT detectors for X-ray polarisation measurement has been around for a while, but this is for the first time that this idea was tested on ground before launch and then employed in space. This makes our measurements much more credible.”
Vadawale is associated to the Physical Research Laboratory of Ahmedabad.
Despite all these preparations, the measurements did not come easy. Professor A R Rao of the Tata Institute of Fundamental Research, the principal investigator of CZTI, said, “Even the brightest sources in the sky give us very few X-ray photons. We had to observe the Crab pulsar many times, and combine data taken many months apart, which was a major challenge, given the very small rotation period of the pulsar.”
To get the micro-second accuracy required 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 the National Centre for Radio Astrophysics (NCRA), Pune, monitored the radio pulsations from Crab with GMRT and Ooty radio telescope.
The full team then spent several months poring over the data, and came up with the best measurements of Crab X-ray polarisation in the world. Most theories predicted that the polarisation of X-ray radiation will show changes during the emission of a pulse, but not at other times. So what does the team make of the surprising observation by CZTI of a sharp change of polarisation in the “off pulse” region?
Professor Dipankar Bhattacharya of IUCAA, Pune, said, “This is a big challenge to theorists.”