A black hole destroys a star and attacks another

A massive black hole tore apart a star and is now using that stellar wreckage to hit another star or smaller black hole that was once in the clear.

This discovery, made with NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, the Neutron Star Interior Composition Explorer (NICER), the Neil Gehrels Swift Observatory and other telescopes, helps astronomers connect two mysteries where previously there were only hints of a connection. The study is published in the journal Nature.

In 2019, astronomers witnessed the signal of a star too close to a black hole and destroyed by the black hole’s gravitational forces. Once shredded, the remains of the star form a disk that rotates around the black hole, like a sort of stellar cemetery.

However, within a few years this disk has expanded outward and is now directly in the path of another star, or perhaps a stellar-mass black hole, orbiting the massive black hole at a previously safe distance. This orbiting star now repeatedly crashes through the debris disk, about once every 48 hours, during its orbit. When this happens, the collision causes bursts of X-rays that astronomers captured with Chandra.

“Imagine a diver repeatedly entering a swimming pool and creating a splash each time he enters the water,” said Matt Nicholl of Queen’s University in Belfast, UK, the lead author of the study. “The star in this comparison is like the diver and the disk is the pool, and every time the star hits the surface it creates a huge “splash” of gas and x-rays. As the star orbits the black hole, she does this again and again.

Scientists have documented numerous cases where an object gets too close to a black hole and is torn apart in a single burst of light. Astronomers call these “tidal disruption events.”

In recent years, astronomers have also discovered a new class of bright flashes coming from the centers of galaxies, which are detected only by X-rays and repeat several times. These events are also linked to supermassive black holes, but astronomers have been unable to explain the cause of the semi-regular X-ray bursts. They have dubbed these “quasi-periodic flares.”

“There has been feverish speculation that these phenomena were linked, and now we have discovered evidence that they are,” said co-author Dheeraj Pasham of the Massachusetts Institute of Technology. “It’s like getting a cosmic double to solve 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 have used Chandra and Hubble to study debris left after the tide. the disruption was over.

The Chandra data were obtained during three different observations, each about 4 to 5 hours apart. Chandra’s total exposure of about 14 hours revealed only a weak signal in the first and last chunks, but a very strong signal in the middle observation.

From there, Nicholl and colleagues used NICER to frequently examine AT2019qiz for repeated X-ray bursts. NICER data showed that AT2019qiz erupts approximately every 48 hours. Observations by Swift and the Indian AstroSat telescope confirmed this discovery.

Hubble ultraviolet data, obtained at the same time as Chandra observations, allowed scientists to determine the size of the disk around the supermassive black hole. They found that the disk had grown large enough that if an object orbited the black hole for a period of about a week or less, it would collide with the disk and cause flares.

“This is a major advance in our understanding of the origin of these regular eruptions,” said Andrew Mummery of the University of Oxford. “We now realize that we have to wait a few years for flares to ignite after a star has been torn apart, because it takes a while for the disk to extend far enough to encounter another star.”

This result has implications for the search for more quasi-periodic eruptions associated with tidal disturbances. Discovering more would allow astronomers to measure the prevalence and distances of objects in close orbit around supermassive black holes. Some of these could make excellent targets for planned future gravitational wave observatories.

NASA missions are part of a growing global network of missions with different but complementary capabilities, monitoring such changes to solve the mysteries of how the universe works.