October 10, 2024

A massive black hole has torn apart one star and is now using that stellar wreckage to pummel another star or smaller black hole that used to be in the clear. This discovery was made using NASA's space observatories—Chandra, HST, NICER, Swift—and IS-RO's AstroSat. It provides astronomers with valuable insights, linking two mysteries where there had previously only been hints of a connection.

In 2019, astronomers witnessed the signal of a star that got too close to a black hole and was destroyed by the black hole’s gravitational forces. Once shredded, the star’s re-mains began circling the black hole in a disk in a type of stellar graveyard.

Over a few years, however, this disk has expanded outward and is now directly in the path of a star, or possibly a stellar-mass black hole, orbiting the massive black hole at a previously safe distance. The orbiting star is now repeatedly crashing through the debris disk, about once every 48 hours, as it circles. When it does, the collision causes bursts of X-rays that astronomers captured with Chandra.

“Imagine a diver repeatedly going into a pool and creating a splash every time she enters the water,” said Matt Nicholl of Queen's University Belfast, United Kingdom, the lead au-thor of the study that appears in the current issue of Nature. “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. Astronomers call these “tidal disruption events” (TDEs). In recent years, astronomers have also discovered a new class of bright flashes from the centers of galaxies, which are detected only in X-rays and repeat many times. These events are also connected to supermassive black holes, but astronomers could not explain what caused the semi-regular bursts of X-rays. They dubbed these “quasi-periodic eruptions,” or QPEs.

“There had been feverish speculation that these phenomena were connected, and now we've discovered the proof that they are,” said co-author Dheeraj Pasham of the Massa-chusetts Institute of Technology. “It's like getting a cosmic two-for-one in terms of solving mysteries.”

This tidal disruption event 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 Chandra and NASA's Hubble Space Telescope to study the debris left behind after the tidal disruption had ended.

The Chandra data were obtained during three different observations, each separated by about 4 to 5 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 observa-tion.

From there Nicholl and collaborators used NASA's Neutron Star Interior Composition Explorer (NICER) to look frequently at AT2019qiz for repeated X-ray bursts. The NICER data showed that AT2019qiz erupts roughly every 48 hours. Observations from NASA''s Neil Gehrels Swift Observatory and India's AstroSat telescope cemented the finding.

The ultraviolet data from Hubble, obtained at the same time as the Chandra observa-tions, allowed the scientists to determine the size of the disk around the supermassive black hole. They found that the disk had become large enough that if any object was or-biting the black hole with a period of about a week or less, it would collide with the disk and cause eruptions.

Gulab Dewangan, a co-author from the Pune-based Inter-University Center for Astron-omy & Astrophysics (IUCAA), noted, India's AstroSat mission provides unique UV/X-ray capability for studying such events. AstroSat's Soft X-ray Telescope and the 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."

“This is a huge breakthrough in our understanding of the origin of these regular eruptions,” said Andrew Mummery of Oxford University. “We now realize we need to wait a few years for the eruptions to ‘turn on’ after a star has been torn apart because it takes some time for the disk to spread out far enough to encounter another star.”

This result has implications for searching for more quasi-periodic eruptions associated with tidal disruptions. Finding more of these would allow astronomers to measure the prevalence and distances of objects in close orbits around supermassive black holes. Some of these may be excellent targets for the planned future gravitational wave ob-servatories.

The paper describing these results is published by Nature in the October 9 issue of the journal. The paper will be available at the URL .

Title: Quasi-periodic X-ray eruptions years after a nearby tidal disruption event

Image caption: Artist's impression of an orbiting star crashing through the accretion disk around a supermassive black hole and causing a burst of X-rays. The disk was cre-ated by the destruction of a previous star. Inset: Detection of X-rays (purple) with NASA's Chandra X-ray Observatory, overlaid on a visible light image of the host galaxy

Readers may contact Prof. Gulab Dewangan, IUCAA, Pune for more information Prof Gulab C Dewangan (gulabd[at]iucaa[dot]in)