E-Paper
Astro Sat’s CZT Imager measures X-ray polarisation of Cygnus X-1 black hole system
The CZTI instrument onboard Astro Sat consists of a large number of small size pixels (around 16,000 in number), each of which works as an independent detector
Astro Sat, India’s first, dedicated multi-wavelength astronomy mission, accomplished the difficult task of measuring the X-ray polarisation of the Cygnus X-1 black hole system from data collected over the last eight years of operation. The measurements have been reported in an article published in the Astrophysical Journal Letters. With these unique measurements from Astro Sat’s CZT (Cadmium Zinc Telluride) Imager (CZTI), scientists have come a step closer to unravelling the mechanism of production of radiation in these sources.
According to professor A R Rao from the Tata Institute of Fundamental Research (TIFR) and PI of the CZTI payload, “X-ray polarisation measurements are critical for astrophysics, and CZTI is uniquely capable of measuring polarisation in this energy range. These results will be used by theoretical astrophysicists around the world for their theories for years to come.”
The CZTI instrument onboard Astro Sat consists of a large number of small size pixels (around 16,000 in number), each of which works as an independent detector. A high energy X-ray photon striking such a detector generates a small electric signal which helps measure the energy of the photon. An incident X-ray photon sometimes undergoes “Compton scattering” that produces a signal in two neighbouring pixels. These pixel pairs tend to align preferably in the direction perpendicular to the polarisation vector of the incident radiation. The CZTI was built to enable the measurement of this effect after extensive tests on ground before launch. In a massive ground-calibration effort, each of the 16,000 pixels of CZTI was calibrated with the utmost detail before the launch of the satellite. A sophisticated firmware developed by the team at Vikram Sarabhai Space Centre, Thiruvananthapuram, played a key role in recording the polarisation signal on board.
While professor Gulab Dewangan from Inter-University Centre for Astronomy and Astrophysics (IUCAA) Pune, who is part of the team, said, “The Indian research community is growing stronger in the field of X-ray polarisation, and the use of an existing instrument for this measurement is a testimony to the ingenuity of our researchers. These results and the experience will be useful when data from XPo Sat is available.”
The lead author of the paper, Dr Tanmoy Chattopadhyay, who works at Stanford University, said, “We have worked hard on this for a decade - starting with my PhD thesis before the launch of Astro Sat. It is very fulfilling to get these wonderful results following all the efforts.”
Despite all these preparations, the measurements did not come easy. The challenge was that Cygnus X-1, despite being one of the brightest sources of X-rays, provides very few hard X-ray photons. The cosmic X-ray background is everywhere, so to measure the polarisation of the actual source photons, it is important to identify and subtract the actual X-ray background. Hence, several months were spent in ensuring the reliability of the results. In the process, novel analysis techniques and statistical tests were developed. Nearly one million seconds of observation were carried out from Astro Sat for this source under the special Astro Sat proposal scheme known as Astro Sat long term key proposals (ALTKP). This was a vast collaborative effort across many academic institutes.
Discovered over four decades ago, Cygnus X-1 is one of the first confirmed black hole systems in our galaxy (located at a distance of 400 million times the distance between the Sun and Earth). The black hole in Cygnus X-1 is 20 times heavier than the Sun, and has a companion - a heavy supergiant star (40 times more massive than the Sun) in a binary system. Due to the gravitational pull of the black hole, material from the supergiant falls and spirals in towards the black hole. This process leads to the formation of a thin accretion disk which is responsible for soft X-rays.
