NEW DELHI: The AstroSat, India’s multi-wavelength space telescope, has successfully accomplished the extremely difficult task of measuring X-ray polarisation. A team of scientists of Indian Space Research Organisation(ISRO) has documented the results of the 18 month study of Crab pulsar in the Taurus Constellation and measured the variations of polarisation as this highly magnetised object spins around 30 times every second. This landmark measurement puts up a strong challenge to prevailing theories of high energy X-ray emission from pulsars. X-ray polarisation measurement is so difficult, that so 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, observed in 1054 AD. Indian scientists using data from the CZT Imager instrument of the AstroSat satellite 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 Cadmium Zink Tellurude Imager (CZTI) instrument is consists of 16384 tiny pixels, essentially independent detectors, to measure X-rays coming from the celestial objects. When high energy X-rays shine on these detectors, an extremely small electric signal is generated, which is further processed to measure the energy of the photon. The incident X-ray photon sometimes leaves a signal in two neighbouring pixels, due to an effect called “Compton scattering”.
“The idea of using pixelated CZT detectors for X-ray polarization 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. Another important feature of our measurements is the study of polarization properties at different rotation phases of the pulsar, which has not been done so far,” says Prof Santosh Vadawale of Physical Research Laboratory(PRL) Ahmedabad, the lead author of this paper, who is a part of the CZTI team and has vast experience of handling CZT detectors at TIFR, Harvard, and later for the Chandrayaan-1 mission at PRL. Despite all these preparations, the measurements did not come easy. Prof. A. R. Rao of Tata Institute of Fundamental Research, the principal investigator of CZTI, says: “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 – a major challenge, given the very small rotation period of the pulsar.” (AGENCIES)
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