Cosmic discovery solves black hole mystery that had perplexed scientists

The new evidence helps astronomers conclusively link 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 remains began circling the black hole in a disc shape in a type of “stellar graveyard”.

Over a few years, however, this disc has expanded outwards 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 disc, about once every 48 hours, as it circles. When it does, the collision causes spectacular light shows and bursts of X-rays that astronomers captured with Chandra.

Lead author Dr Matt Nicholl, of Queen’s University Belfast, said: “Imagine a diver repeatedly going into a pool and creating a splash every time she enters the water.

“The star in this comparison is like the diver and the disc 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 long 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 additionally 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 could not explain what caused the semi-regular bursts of X-rays. They dubbed these “quasi-periodic eruptions”, or QPEs.

Co-author Dr Dheeraj Pasham, of the Massachusetts Institute of Technology, said: “There had been feverish speculation that these phenomena were connected, and now we’ve discovered the proof that they are.

“It’s like getting a cosmic two-for-one in terms of solving mysteries.”

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.

From there, Dr Nicholl and team used Nasa’s Neutron Star Interior Composition Explorer (Nicer) to look frequently for repeated X-ray bursts.

Ultraviolet data from the Hubble space telescope, obtained at the same time as the Chandra observations, allowed the scientists to determine the size of the disc around the supermassive black hole.

They found that the disc had become large enough so that, if any object was orbiting the black hole with a period of about a week or less, it would collide with the disc and cause eruptions.

Co-author Dr Andrew Mummery, of Oxford University, said: “This is a huge breakthrough in our understanding of the origin of these regular eruptions.

“We now realise 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 disc to spread out far enough to encounter another star.”

This result has implications for searching for more QPEs 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 targets for the planned future gravitational wave observatories.

Nasa’s Marshall Space Flight Centre manages the Chandra programme. The Smithsonian Astrophysical Observatory’s Chandra X-ray Centre controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

– The research paper will appear in the October issue of Nature.