On the brink: Mysterious object resists being swallowed by a black hole

ESA’s XMM-Newton X-ray telescope has detected fluctuations in X-ray emission coming from a supermassive black hole at the center of a nearby galaxy. The obtained results paint a fascinating picture that refutes our ideas about how matter falls into black holes and points to a potential source of gravitational waves.

Fluctuations of X-ray radiation

During the fall into a black hole, the doomed matter forms a disk around it. This process is called accretion. The gas in the accretion disk is heated and emits mainly ultraviolet radiation. It interacts with the cloud of electrically charged gas surrounding the black hole, called the corona. During this interaction, ultraviolet rays gain energy, boosting them to X-rays.

XMM-Newton X-ray telescope (concept). Source: ESA-C. Carreau

The attention of researchers was drawn by the XMM-Newton telescope recorded oscillations of X-ray radiation emitted by the black hole 1ES 1927+654. In 2018, it experienced a large flare that seems to have disturbed its surroundings because the X-ray corona disappeared. Gradually the corona returned and normalcy was restored by early 2021.

Object on the edge of black hole

However, in July 2022, XMM-Newton began to observe that the X-ray power of 1ES 1927+654 varies by about 10% on time intervals between 400 and 1000 seconds. Such oscillations indicate that there is some massive object in the accretion disk. As the object approaches the black hole, the orbital travel time decreases, resulting in an increase in the oscillation frequency. 

A white dwarf in orbit around a black hole in an artist’s impression. Source: NASA/Sonoma State University, Aurore Simonnet

Calculations have shown that this mysterious object is probably a white dwarf, with a mass about 0.1 times that of the Sun, moving at an incredible speed. It made one orbit around the hole, covering a distance of about 100 million kilometers in 18 minutes.

For nearly two years, XMM-Newton has shown that the fluctuations are getting stronger and more frequent. The researchers hypothesized that the object’s orbital energy is radiated as gravitational waves, according to the general theory of relativity. And this meant that it should be even more strongly attracted to the black hole and be absorbed by it in the near future.

Mysterious rescue

To test this idea, they calculated when this object would cross the event horizon and disappear, after which the oscillations would stop. It turned out to be on January 4, 2024. However, in March 2024, XMM-Newton again registered oscillations. This meant that the astronomers’ prediction was not justified. So the object traveled at about half the speed of light and made one orbit around the black hole in just seven minutes, stubbornly refusing to be absorbed.

Supermassive black hole (simulation). Source: NASA

As a result, the researchers faced the situation that the matter was either not only gravitational waves at all, or the whole hypothesis had to be changed. They considered the idea that the cloud itself around the black hole could be oscillating. However, since there is no established theory to explain this behavior, they went back to the original scenario. They hypothesize that while the black hole’s gravity is pulling the white dwarf in, it is also dumping some of its outer layer into the black hole. This reset acts as a small recoil by which the white dwarf can resist crossing the event horizon.

Whether this is true or not will be known in the 2030s when ESA launches the LIST gravitational observatory. If there really is a white dwarf in orbit around the black hole, it will have to see it.

According to ESA

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