IIT Guwahati, ISRO study finds X-ray polarisation in black hole

Guwahati: In a first, researchers from the Indian Institute of Technology Guwahati (IIT Guwahati) and the Indian Space Research Organisation (ISRO) have detected polarised emissions from a black hole source that exists beyond our Milky Way Galaxy.

The findings, published in the Monthly Notices of the Royal Astronomical Society: Letters, open a new window to investigate and understand the nature of astrophysical black hole sources.

Large Magellanic Cloud X-3 (LMC X3) is a binary star system consisting of a black hole and a ‘normal’ star that is much hotter, bigger, and more massive than the Sun. It is located in a satellite galaxy of our Milky Way, nearly 200,000 light-years away from Earth.

Since its discovery in 1971, it has been observed by various satellites. However, there has been a gap in understanding the polarisation properties of X-rays emitted by highly energetic objects like stellar mass black holes in the universe.

The team detected the emissions through a technique called X-ray polarimetry.

“X-ray polarimetry is a unique observational technique to identify where radiation comes from near black holes. LMC X-3 emits X-rays that are 10,000 times more powerful than those from the Sun," said Prof Santabrata Das, Department of Physics, IIT Guwahati, in a statement.

"When these X-rays interact with the material around black holes, specifically when they scatter, it changes the polarisation characteristics, i.e., degree and angle. This helps in understanding how matter is drawn toward black holes in the presence of intense gravitational forces," he added.

The researchers studied LMC X-3 using the Imaging X-ray Polarimetry Explorer (IXPE), the first mission of NASA to study the polarisation of X-rays from celestial objects. They also made use of the simultaneous broad-band coverage of Neutron Star Interior Composition Explorer (NICER) Mission and Nuclear Spectroscopic Telescope Array (NuSTAR) Mission to constrain the spin of LMC X-3.

“Intense gravitational fields can cause the emitted light from black holes to become polarised. Our observations indicate that LMC X-3 likely harbours a black hole with low rotation rate, surrounded by a slim disc structure that gives rise to the polarised emissions,” said Dr. Anuj Nandi, Scientist, from U. R. Rao Satellite Centre (URSC) at ISRO, Bengaluru.