When David comes by, Goliath gets a cough

Astronomers and astrophysicists are familiar with the phenomenon that a luminous object in the sky darkens at regular intervals. At least when the luminous object is a star. The regular darkening is usually an unmistakable sign that a planet is orbiting around this star and, at periodic intervals, is briefly allowing slightly less light from the star to pass through in the direction of the telescope.

But what can noticeably darken the gigantic glow of an entire galaxy at regular intervals? Dheeraj Pasham from the Massachusetts Institute of Technology (MIT) has been keen to get to the bottom of this question ever since he began studying a so-called “Tidal Disruption Event” (TDE) in a galaxy far, far away.

In December 2020, such a TDE was discovered in a galaxy about 800 million light-years away. This part of the sky had been relatively quiet and dark until the telescopes discovered it, when the galaxy suddenly brightened by a factor of 1,000. Dheeraj Pasham decided to observe the eruption using NASA’s NICER (“Neutron star Interior Composition Explorer”), an X-ray telescope on board the International Space Station that continuously monitors the sky for X-ray bursts.

The outbreak lasted about four months before subsiding. During this time, NICER took daily, high-frequency measurements of the galaxy’s X-ray emissions. As Pasham looked more closely at the data, he noticed a strange pattern: subtle dips in a very narrow band of X-rays that appeared every eight and a half days. “I was racking my brain about what this meant, because this pattern doesn’t fit anything we know about these systems,” Pasham remembers.

While looking for an explanation for the periodic dips, Pasham came across a recent paper by theoretical physicists from the Czech Republic. Those Czech physicists had separately discovered that it is theoretically possible for a galaxy’s central supermassive black hole to have a second, much smaller black hole with it. This smaller black hole does not have to orbit its “big brother” at the same angle as the accretion disk.

Pasham sent the Czechs an email in which he wrote: “What your theory predicts is probably what I’m observing here right now.” From then on, Czechs and Americans worked together, and researchers from Italy were also involved in the large project. Their final explanation of what happened and what was seen adds a whole new chapter to the book of black hole knowledge that could be titled “David and Goliath System.”

The researchers suspect that in December 2020, a nearby star came too close to the David and Goliath system and was torn to pieces by Goliath’s immense gravity. The sudden influx of stellar material caused the black hole’s accretion disk to brighten as the star’s debris spun into the black hole. For four months, Goliath “fed” on the stellar debris while David continued to orbit the disk.

The small black hole periodically penetrated the bright accretion disk and released a gas cloud there. Because this cloud pointed in the direction of the observation telescope, a dip in the total energy of the galaxy could be seen; the light was repeatedly briefly blocked. “This is something completely new,” says Dheeraj Pasham. “It doesn’t fit with anything we know about these systems. We see evidence of objects entering and passing through the disk at different angles, challenging the traditional picture of a simple gaseous disk around black holes.”

By the way, the terms “David” and “small” are to be viewed relatively in such a system of black holes. According to the scientists, even “little David” in the scenario examined weighs between 100 and 10,000 solar masses. But that is still very, very little compared to the big black hole. This has a gravity that scientists estimate is 50 million times greater than that of our sun. Somehow reassuring that Goliath is 800 million light years away.