Quasars, identified in the 1960s, rank among the brightest known cosmic sources and are powered by supermassive black holes that draw in surrounding matter. As infalling material forms a rapidly rotating accretion disc and spirals toward the black hole, friction heats the gas to high temperatures and produces intense ultraviolet light, often hundreds to a thousand times brighter than a typical galaxy of 100 billion stars and capable of drowning out the host galaxy's starlight.

This ultraviolet emission is thought to power the X rays seen from quasars, as UV photons pass through a compact region of energetic particles near the black hole known as the corona. When UV photons scatter off these high energy particles, they gain energy and emerge as strong X ray radiation detectable with modern instruments.

Because both components arise from the same accretion flow, quasar X ray and ultraviolet luminosities show a tight correlation, where higher UV brightness usually comes with stronger X ray output. Established almost five decades ago, this relation has underpinned models of the geometry and physical state of gas close to supermassive black holes and has been central to efforts to use quasars as distance indicators.

The new study, led by researchers at the National Observatory of Athens and published in Monthly Notices of the Royal Astronomical Society, finds that this X ray to UV relation is not the same at all epochs. The team reports that when the universe was about half its current age, the correlation between quasar X ray and ultraviolet emission differed significantly from that seen in nearby quasars, implying that the disc corona connection has evolved over roughly the last 6.5 billion years.

"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. "We tested the result using different approaches, but it appears to be persistent."

To probe this evolution, the astronomers combined new observations from the eROSITA X ray telescope with archival measurements from the European Space Agency's XMM-Newton observatory, assembling an unusually large quasar sample. The broad, uniform sky coverage from eROSITA was crucial, allowing the group to examine quasar populations on scales not previously accessible.

The long assumed universality of the UV X ray relation is also a key ingredient in techniques that treat quasars as standard candles to map cosmic expansion and investigate dark matter and dark energy. The reported evolution means that methods assuming a fixed relation over time may be biased, and the structure of material around black holes must be reassessed before drawing firm cosmological conclusions.

"The key advance here is methodological," said postdoctoral researcher Maria Chira of the National Observatory of Athens, who led the work. "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 eROSITA all sky scans will provide even larger samples of faint and distant quasars, allowing researchers to test whether the observed change in the X ray UV relation reflects a genuine shift in accretion physics or arises from selection effects. Analyses that merge upcoming X ray catalogues with multiwavelength surveys should clarify how supermassive black hole discs and coronae evolve and refine the use of quasars in precision cosmology.

Such work will deepen understanding of how black holes drive the most luminous persistent sources in the universe and how their energy output has varied across cosmic history.

Research Report:Revisiting the X-ray-to-UV relation of Quasars in the era of all-sky surveys

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