Study of the X-ray light flickering of quasars —supermassive black holes actively devouring matter— reveals powerful winds of outflowing material blasted away from the system and opens new opportunities for learning about the fierce and dynamic conditions in the inner regions of rapidly growing black holes.
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 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.
Picture 1: artistic representation of matter spiraling onto a supermassive black hole under the influence of gravity.
The light from Quasars has a special feature: it constantly flickers, or varies in brightness, over time. This is because 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. 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. A new study published in Monthly Notices of the Royal Astronomical Society challenges that established rule.
The team, led by researchers at 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, we found that the X-ray flickering actually becomes stronger for black holes that are accreting matter at a faster rate“, said Dr. Antonis Georgakakis from the National Observatory of Athens, the lead author of the study. “This new 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 lies 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.
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 discovery was made possible by the eROSITA X-ray telescope operated by a consortium of German astronomy institutes. eROSITA is designed to repeatedly survey 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 co-author Johannes Buchner, of the Max-Planck Institute of Extraterrestrial Physics.
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.”
IAASARS researchers involved in this work are Drs. Antonis Georgakakis, Angel Ruiz and Maria Chira. The research leading to these results is funded from the Hellenic Foundation for Research and Innovation (HFRI) project “4MOVE-U” grant agreement 2688, which is part of the programme “2nd Call for HFRI Research Projects to support Faculty Members and Researchers”.