Relativistic TDE

Caught in the act: Indian Astronomers study a Rare Luminous Jet Spewed by Supermassive Black Hole

Members of the GROWTH-India collaboration working with an international team studied a bright optical flare caused by a dying star’s encounter with a supermassive black hole.

What happens when a dying star flies too close to a supermassive black hole?

“It doesn’t end well for the star”, says Varun Bhalerao, an astrophysicist at IIT Bombay. “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.”

Indian astronomers using the GROWTH-India Telescope (GIT) recently studied a very rare TDE, where the black hole took part of the stellar material and launched it as “relativistic jets” - beams of matter travelling close to the speed of light. The results of this study, jointly with a large international team, were published in the journal Nature on November 30, 2022.

Artist’s rendering showing the Tidal Disruption of a star by a supermassive black hole, leading to the creation of jets.Image credits: Carl Knox (OzGrav, ARC Centre of Excellence for Gravitational Wave Discovery, Swinburne University of Technology)
Data from Zwicky Transient Facility and other international collaborators showed that AT2022cmc was a “transient” - an object that is visible for a short time in the sky. The collaboration finds hundreds of such transients daily - and most are rather uninteresting. Data from the GROWTH-India telescope (blue points) in the few days following the discovery showed the peculiar fading rate of AT2022cmc. This was the key clue that got the team interested in studying it in great detail, leading to a fascinating discovery!Image Credits: Harsh Kumar / GROWTH-India Team

The story started with images obtained by the California-based Zwicky Transient Facility project on 11 February 2022. These showed a new source in the sky, dubbed AT2022cmc, which seemed to have brightened rapidly and was now fading fast. “We immediately jumped into action and started obtaining daily observations with the GROWTH-India Telescope”, says 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”.

Harsh and the Indian team brought this to the notice of their international collaborators, including University of Maryland astronomer Dr. Igor Andreoni. “The GROWTH India data showed us that the source was special.” The team then concentrated their resources to understand the mystery object - obtaining observations from Radio telescopes, X-ray telescopes, large ground-based optical telescopes, and even the Hubble Space Telescope. Dr. Andreoni adds, “Without GROWTH-India observations, we would probably not have undertaken these observations which revealed the extreme nature of this object.”

Follow-up observations with many observatories confirmed that AT2022cmc was fading rapidly and the ESO Very Large Telescope revealed that AT2022cmc was at a cosmological distance, 8.5 billion light years away: the farthest TDE ever seen.

Hubble Space Telescope optical/infrared images and radio observations from the Very Large Array pinpointed the location of AT2022cmc with extreme precision. The researchers believe that AT2022cmc was at the centre of a galaxy that is not yet visible because the light from AT2022cmc outshone it, but future space observations with Hubble or James Webb Space Telescopes may unveil the galaxy when the transient eventually disappears.

Artist’s concept showing the relativistic jet created by the tidal disruption of a star by a supermassive black hole. The annotations show how light of various wavelengths is produced by such an event.Image credit: Zwicky Transient Facility / R.Hurt (Caltech/IPAC)

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 these jets. The last such discovery had been made a decade ago! This was the first discovery in optical - and it also turned out to be the farthest TDE ever found. “Such remarkable results can only be obtained when large international teams pool their resources and coordinate their activities”, said G. C. Anupama, a visiting professor at the Indian Institite of Astrophysics (IIAP), Bangalore.

It is still a mystery why some TDEs launch jets while others do not seem to. From their observations, Andreoni and his team concluded that the black holes in AT2022cmc and other similarly jetted TDEs are likely spinning rapidly so as to power the extremely luminous jets. This suggests that a rapid black hole spin may be one necessary ingredient for jet launching—an idea that brings researchers closer to understanding the physics of supermassive black holes at the centre of galaxies billions of light years away. 

How will we know for sure? Sudhanshu Barway, GIT Facility Scientist at IIAP says, “By finding more such events. The universe is full of exotic, puzzling events. Whatever the universe sends our way next, the GIT team is ready!”

Small telescope, big punch!The GROWTH-India Telescope (GIT) is among the smaller research telescopes in the world, yet it packs a big punch. It is India’s only fully robotic telescope. Every night, the telescope obtains data for dozens of different objects, and automated pipelines rapidly process the data to create science-ready data products. This makes GIT invaluable for the international GROWTH collaboration.

Publication:

The paper, “A very luminous jet from the disruption of a star by a massive black hole,” was published in Nature on November 30, 2022.

Telescopes involved in this work: Palomar 48-inch Samuel Oschin Telescope (Zwicky Transient Facility), Liverpool Telescope, Blanco telescope, GROWTH-India Telescope, Very Large Telescope, Nordic Optical Telescope, Very Large Array, Submillimeter Array, Northern Extended Millimeter Array, James Clerk Maxwell Telescope, upgraded Giant Metrewave Radio Telescope, Palomar 60-inch Telescope, Palomar 200-inch Hale Telescope, Asteroid Terrestrial-impact Last Alert System, Gran Telescopio Canarias, Calar Alto Observatory, W.M. Keck Observatory, Gemini Observatory, Neil Gehrels Swift Observatory, Neutron Star Interior Composition Explorer and Hubble Space Telescope.

The GROWTH India Telescope is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the telescope’s operations. Telescope technical details are available at https://sites.google.com/view/growthindia/ . 

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Caught in the act: Indian Astronomers study a Rare Luminous Jet Spewed by a Supermassive Black Hole

December 2022

The GROWTH-India team obtained key data that helped in the identification and study of a rare, relativistic jetted tidal disruption event.