eROSITA Reveals Powerful Black Hole Winds Driving Unexpected X-ray Flickering in Quasars

The eROSITA X-ray telescope operated by a Consortium led by the Max Planck Institute of Extraterrestrial Physics (MPE) made a surprising discovery concerning Quasars—supermassive black holes actively devouring matter. Contrary to previous expectations the new eROSITA study found that fast-feeding black holes demonstrate stronger X-ray flickering and more erratic variations of their X-ray brightness with time. MPE researchers and an international team of astronomers leading the study propose that this unexpectedly strong variability is caused by powerful winds of material blasting away from the black hole itself, which act like a "cosmic curtain", intermittently blocking the X-ray light we observe. MPE had key contributions to this discovery in building eROSITA which systematically scans the sky every 6 months, and laying the foundation to new statistical analyses revealing patterns in the large volumes of data.

Most massive galaxies in the universe, including our own Milky Way, host at their centres black holes that weigh millions or even billions of times the mass of our Sun. The strong gravitational pull of these beasts captures matter, mostly gas, making it spiral inward and eventually fall in them, growing their masses even more.

Matter however, does not just disappear quietly. The intense gravity and friction heat it up to incredible temperatures, generating huge amounts of energy that is radiated away into space as brilliant light. It is this light that our telescopes, like eROSITA, can see. When a supermassive black hole is actively feeding and generating this powerful light show, we call it a Quasar. These objects are some of the brightest in the universe.

The light from Quasars has a special feature: it constantly flickers, or varies in brightness, over time. “The flow of matter onto the black hole is not perfectly steady. Random changes in the amount of material being eaten cause the Quasar's light to flicker.”, explains Dr. Johannes Buchner, researcher at MPE.
Studying this flickering helps astronomers map the environment close to the black hole and understand how it consumes matter.

Previous research had found a clear pattern: the more massive the black hole, or the faster it feeds, the weaker the flickering of the emitted radiation was expected to be. The eROSITA observations challenge this established rule.

In a new study published in Monthly Notices of the Royal Astronomical Society, MPE astronomers and their international partners led by the National Observatory of Athens used new observations from the eROSITA X-ray telescope to monitor the time variations of the X-ray light from an unprecedented number of distant Quasars.

Contrary to previous expectations, they found that the X-ray flickering actually becomes stronger for black holes that are accreting matter at a faster rate. This unexpected result tells us something fundamental about the flow of matter around rapidly-growing black holes. 

So, why the unexpected strong flicker? The astronomers propose that the answer might lie in powerful winds of outflowing material produced by the Quasar itself. “The environment around rapidly growing black holes is thought to be violent and highly dynamic. The energy released as matter spirals into the black hole is so immense that it actually pushes some of the material away, creating powerful "winds" of gas and dust that blast outward. These winds are blowing out even as new material is spiraling in.”, explains lead researcher of the study, Dr. Antonis Georgakakis from the National Observatory of Athens.

The ejected matter may act as a cloud or curtain, temporarily blocking the X-ray light generated by the accretion process and leading to the erratic X-ray flux variations we observe with our telescopes. This discovery offers a new way to study the fierce conditions in rapidly growing black holes.

The key feature of eROSITA that made this discovery possible is that it surveys the entire sky every six months. This strategy provides the most sensitive X-ray observations ever over such vast areas. Since its launch, eROSITA has completed four full sky scans, providing astronomers with an unparalleled time-lapse view of the cosmos.

“These repeat observations track changes over time of the X-ray light intensity emitted by individual Quasars, making eROSITA the perfect tool to study the X-ray flickering as supermassive black holes devour matter”, reveals Dr. Mara Salvato, eROSITA spokesperson and co-author of the study.

The international team developed novel statistical methods to analyse the eROSITA data and make the most of these exceptional quality observations. 

“Often, a source's X-ray flux, as it changes over time, drops so low that it falls below eROSITA’s sensitivity limits, becoming effectively invisible and giving us only an upper limit of the flux rather than a precise measurement,” explains Maurizio Paolillo, professor at the University of Naples Federico II and co-author of the study. “Traditional analysis methods usually ignore this 'hidden' information. Our new statistical approach is specifically designed to exploit these upper limits, significantly improving our constraints on the variability properties of Quasars.”

The new discovery opens new opportunities to study the fierce conditions in rapidly growing black holes. The team will also investigate other hypotheses that could cause the additional flickering.