Ladakh telescope, Indian astronomers play key role in rare Black Hole discovery

A Himalayan telescope in Ladakh and a group of Indian astronomers have alerted the world about the death screams of a dying star, which was torn apart by a supermassive Black Hole at a distance of 12.5 billion light years away - more than halfway across the Universe.

Though the rare cosmic event that generated the most powerful flash from the farthest ever detected – it is more than 1,000 trillion times more luminous than the Sun – was observed by a network of telescopes in four continents and from space, it was the GROWTH-India telescope at Hanle that gave the first heads-up on the unusual nature of the flash to the astronomy community around the world within days of it being spotted.

The story began with California-based Zwicky Transient Facility detecting a new source of a bright flash in the sky in the second week of February. Named AT2022cmc, it brightened rapidly and was fading fast.

“We immediately jumped into action and started obtaining daily observations with the GROWTH-India Telescope”, said Harsh Kumar, a PhD student at IIT Bombay. “Our data showed that the object was fading at a unique, unexpected rate that set it apart from dozens of other sources we study daily”.

This led to a string of follow-up observations by more than 20 telescopes around the world and in the sky including India’s GMRT and Astrosat observatory.

The astronomers observed the last tango of a dying star, which was being gobbled by a supermassive black hole, giving them ideas on what happens when a dying star flies too close to a supermassive Black Hole.

“It doesn’t end well for the star”, said Varun Bhalerao, an astrophysicist at IIT Bombay and a team member. “The star gets violently pulled apart by the black hole’s gravitational tidal forces. The shreds of the star form a spinning disc around the black hole, and are eventually consumed by it. Such events are called Tidal Disruption Events, or TDEs.”

Before AT2022cmc, the only two previously known jetted TDEs were discovered through gamma-ray space missions, which detect the highest-energy forms of radiation produced by such jets. The last such discovery was a decade ago.

Since the Big Bang happened 13.8 billion years ago, what scientists observed happened in a young universe. “It is hard to estimate the details of the star that died (it became bright only because it was already torn apart), but it was probably a normal star, perhaps similar to even the mass of the Sun. Also, it did something weird," Bhalerao told DH.

A part of the stellar material was released as “relativistic jets” - beams of matter travelling close to the speed of light -directed towards the Earth.

“Our alerts led other astronomers to carry out follow-up observations,” said G C Anupama, former dean of the Indian Institute of Astrophysics, Bengaluru. Two Pune-based groups from IUCAA and NCRA were also a part of the study.

India’s uGMRT and Astrosat, as well as the VLA and Hubble Space Telescopes, were among the instruments that were used to study the celestial event. The results appeared in two research papers in Nature and Nature Astronomy.

“The GROWTH India data showed us that the source was special. Without such data we would probably not have undertaken these observations which revealed the extreme nature of this object,” said Igor Andreoni, University of Maryland astronomer and one of the lead authors of the study.