Recent findings from an international team of astronomers indicate that the relationship between ultraviolet and X-ray emissions in quasars has changed significantly over billions of years. This unexpected discovery suggests that the material surrounding supermassive black holes is not static, challenging long-held beliefs in astronomy.
The study, led by researchers at the National Observatory of Athens, was published in the Monthly Notices of the Royal Astronomical Society on December 27, 2025. If these results hold true, they could overturn a foundational principle that has guided astronomical research for nearly half a century.
Understanding Quasars and Their Brightness
Quasars are among the brightest objects in the universe, first identified in the 1960s. They derive their extraordinary luminosity from supermassive black holes that consume surrounding matter. As this material spirals inwards, it forms a rotating disk that generates immense heat through friction, causing it to emit between 100 and 1,000 times more light than an entire galaxy comprising approximately 100 billion stars. This remarkable brightness enables quasars to outshine their host galaxies and be visible across vast cosmic distances.
The intense disk around a black hole produces copious amounts of ultraviolet light, which is critical in the generation of even more energetic X-rays. When ultraviolet rays travel outward, they interact with highly energized particles in a region known as the “corona,” gaining energy and transforming into powerful X-ray radiation. These X-rays can then be detected by space-based observatories.
Challenging Long-Standing Assumptions
Traditionally, astronomers have understood a close relationship between the ultraviolet and X-ray emissions from quasars. Brighter ultraviolet light typically correlates with stronger X-ray output, a connection identified nearly five decades ago. The new study questions the universality of this relationship, suggesting that the structure around black holes may vary across cosmic time.
The researchers found that when the universe was younger, approximately half its current age, the relationship between ultraviolet and X-ray emissions appeared distinctly different from what is observed in nearby quasars today. This change indicates a potential evolution in how the accretion disk and corona interact over the last 6.5 billion years. Dr. Antonis Georgakakis, one of the authors, remarked, “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.”
To arrive at their conclusions, the team combined recent X-ray observations from the eROSITA X-ray telescope with archival data from the XMM-Newton X-ray observatory. This comprehensive approach allowed scientists to analyze the emissions of a large sample of quasars, benefiting from the broad and consistent sky coverage provided by eROSITA.
The implications of these findings extend into cosmology, as the assumption of a stable environment surrounding black holes is integral to certain methodologies that use quasars as “standard candles” for mapping the universe and exploring concepts such as dark matter and dark energy.
Maria Chira, a postdoctoral researcher leading the study, emphasized the methodological advance: “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.”
Future observations from upcoming eROSITA all-sky scans are anticipated to reveal even fainter and more distant quasars. By integrating these observations with next-generation X-ray and multiwavelength surveys, researchers aim to discern whether the observed changes represent genuine physical evolution or are influenced by data collection methods. These efforts promise to deepen our understanding of how supermassive black holes drive the most luminous objects in the universe and how their behavior has evolved over cosmic time.
