NASA & Astro Sat capture dramatic eruptions from stellar wreckage around massive black hole

The discovery has provided astronomers with valuable insights, linking two mysteries where there were only hints of a connection previously

An international group of astronomers, including astronomers from the Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA), has recently gained valuable insights linking two important mysteries related to the Universe. The research involves a ‘star’ repeatedly crashing through the debris disk of another destroyed star about once every 48 hours as it orbits around a bigger black hole. The collision causes quasi-periodic bursts of X-rays to be released from the source called AT2019qiz. This may in turn be related to a newly discovered class of bright flashes from the centres of galaxies, which are also detected only in X-rays and repeat many times, but the reason is not known. India’s Astro Sat telescope played a role in confirming this finding.

The research involves a ‘star’ repeatedly crashing through the debris disk of another destroyed star about once every 48 hours as it orbits around a bigger black hole. The collision causes quasi-periodic bursts of X-rays to be released from the source called AT2019qiz. (REPRESENTATIVE PHOTO)

Using NASA’s space observatories—Chandra, HST, NICER, Swift—and ISRO’s Astro Sat, it was discovered that a massive black hole has torn apart a star and is now using its stellar wreckage to pummel another nearby star (or smaller black hole) which used to be in the clear earlier. The discovery has provided astronomers with valuable insights, linking two mysteries where there were only hints of a connection previously.

In 2019, astronomers witnessed the signal of a star that got too close to a black hole and was destroyed by its gravitational force. Once shredded, the star’s remains began circling the black hole in a disk, something akin to a stellar graveyard. Over a few years, this disk expanded outward and is now directly in the path of a star (or possibly a stellar-mass black hole) which was orbiting the massive black hole at a safe distance previously. The orbiting ‘star’ is now repeatedly crashing through the debris disk, about once every 48 hours, as it circles around the bigger black hole. When it does so, the collision causes bursts of X-rays to be released, which astronomers captured with the Chandra X-ray Observatory.

Matt Nicholl of Queen’s University, Belfast, United Kingdom, who is the lead author of the study published in the current issue of the ‘Nature’ journal, said, “Imagine a diver repeatedly going into a pool and creating a splash every time s/he enters the water. The star in this comparison is like the diver and the disk is the pool, and each time the star strikes the surface, it creates a huge ‘splash’ of gas and X-rays. As the star orbits around the black hole, it does this over and over again.”

Scientists have documented many cases where an object gets too close to a black hole and gets torn apart in a single burst of light. These are called ‘tidal disruption events’ or TDEs. In recent years, astronomers have also discovered a new class of bright flashes from the centres of galaxies which are detected only in X-rays and repeat many times. These events are also connected to supermassive black holes, but astronomers have not been able to explain what causes the semi-regular bursts of X-rays. They have dubbed these as ‘quasi-periodic eruptions’ or QPEs.

Co-author of the study Dheeraj Pasham of the Massachusetts Institute of Technology, said, “There had been feverish speculation that these phenomena are connected, and now we have discovered proof that they are. It’s like getting a cosmic two-for-one in terms of solving mysteries.”

This TDE, now known as AT2019qiz, was first discovered by a wide-field optical telescope at the Palomar Observatory, called the Zwicky Transient Facility, in 2019. In 2023, astronomers used both NASA’s Chandra and Hubble Space Telescope to study the debris left behind after the TDE ended. The Chandra data was obtained during three different observations, each separated by about four to five hours. The total exposure of about 14 hours of Chandra time revealed only a weak signal in the first and last chunk, but a very strong signal in the middle observation.

Gulab Dewangan, co-author of the study from the IUCAA, said, “India’s Astro Sat mission provides unique UV/X-ray capability for studying such events. Astro Sat’s Soft X-ray Telescope and Ultra-Violet Imaging Telescope (UVIT) both detected the source AT2019qiz, but the eruptions were only seen in X-rays. Future sensitive simultaneous X-ray and UV observations of similar eruptions will enable a deeper investigation into their nature.”

The paper describing these results, published in the October 9 issue of ‘Nature’, is available at https://www.nature.com/articles/s41586-024-08023-6.