An international team of astronomers has found evidence that the material around supermassive black holes has evolved over billions of years, challenging long-held assumptions. Observations of quasars show a shifting relationship between ultraviolet and X-ray emissions from the early universe to the present. The discovery, led by researchers at the National Observatory of Athens, suggests that black hole environments are not static across cosmic history.
Quasars, among the brightest objects in the universe, are fueled by supermassive black holes that accrete surrounding matter. This process creates a hot, rotating disk that emits intense ultraviolet light. Scientists have long believed that this ultraviolet radiation interacts with a nearby region called the corona, producing powerful X-rays. For nearly 50 years, the link between these emissions has been seen as a universal constant, providing insights into conditions near black holes.
However, new research published in Monthly Notices of the Royal Astronomical Society indicates this relationship has changed. By analyzing data from the eROSITA X-ray telescope and the European Space Agency's XMM-Newton observatory, the team examined a large sample of quasars. They discovered that about 6.5 billion years ago, when the universe was roughly half its current age, the ultraviolet-to-X-ray connection differed markedly from what is observed in nearby quasars today.
"Confirming a non-universal X-ray-to-ultraviolet relation with cosmic time is quite surprising and challenges our understanding of how supermassive black holes grow and radiate," said Dr. Antonis Georgakakis, one of the study's authors. The findings persisted across multiple analytical methods, pointing to potential evolution in the accretion disk and corona structures.
Led by postdoctoral researcher Maria Chira, the study leveraged eROSITA's broad sky coverage and a Bayesian statistical framework to detect subtle trends in sparse data. "The key advance here is methodological," Chira noted. "The eROSITA survey is vast but relatively shallow—many quasars are detected with only a few X-ray photons. By combining these data in a robust Bayesian statistical framework, we could uncover subtle trends that would otherwise remain hidden."
These results have implications for cosmology. Quasars are often used as standard candles to map the universe's expansion and probe dark matter and energy. If their emission properties vary over time, astronomers must refine these methods. Future eROSITA scans and multiwavelength surveys could clarify whether the changes reflect physical evolution or observational biases, offering deeper understanding of black hole behavior across cosmic epochs.