AstroSat detects polarised high energy X-rays for first time from a black hole

Normal X-ray measurements only reveal the energy or intensity of the radiations. Polarisation characteristics of X-rays, which tells scientists about the orientation of the oscillating electric field, holds clues on the geometry and other properties of the black hole.

“It was an extremely challenging task and many Indian institutes contributed to this work,” lead author of the paper, Dr Tanmoy Chattopadhyay, who works at Stanford University told The Indian Express.

With these measurements made by the Cadmium Zinc Telluride Imager (CZTI), one of the five scientific instruments on board Astrosat, scientists have inched closer to unravelling the mechanism of production of radiation in these sources, the statement said.

Emissions of high-energy X-rays from a black hole jet, like in Cygnus X-1, is an open question in the astrophysics community.

“This study for the first time connected the hard X-ray radiation to the black hole jet. We have worked hard on this for a decade – starting with my PhD thesis before the launch of AstroSat. It is very fulfilling to get these wonderful results following all the effort,” Dr Chattopadhyay said.

Prof Gulab Dewangan from IUCAA, who is part of this discovery, said that polarization is one of the properties that a photon carries from the celestial objects in the sky to us (energy, time, and location of emission are the other three parameters).

“Polarisation of light or electromagnetic waves tells us about the orientation of the oscillating electric field — which in turn is decided by the accelerating charged particle emitting the light. Measurement of polarisation in X-ray wavelengths of light is an extremely difficult task,” he said.

But this information can tell us a lot about what is going on at the close vicinity of the black hole, he said.

Scientists invested almost 10-12 years to develop necessary tools to use the data from Indian astronomical observatory ‘AstroSat’ and measure polarization of Cygnus X-1 in X-ray. Astrosat was launched by Indian Space Research Organisation (ISRO) in September 2015.

“We measured high polarization in this source in the 100-380 keV (a measure of energy) range which implies that radiation emitted from the source at energies above 100 keV, is likely to have originated at the black hole jet,” Dr Chattopadhyay explained.

Discovered over four decades ago, Cygnus X-1 is one of the first confirmed black hole systems in our galaxy, located at a distance that is about 400 times more than the distance between Earth and Sun.

Cygnus X-1 is twenty times heavier than the Sun, and has a companion — a heavy supergiant star (forty 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, Prof Dewangan said.