For more than two decades, the quasar PHL 1811 has been considered the prototype of a rare class of “intrinsically X-ray weak” quasars, thought to produce unusually little X-ray radiation. But in 2024, the Einstein Probe spacecraft caught the object in a bright X-ray flare. In a new study, astronomers combined that observation with more than 20 years of archival data to revisit the mystery. The findings were published in The Astrophysical Journal on July 28.
Mysterious X-ray weakness
In active galactic nuclei (AGN), material spiraling into the central black hole releases enormous amounts of energy. The accretion disk—the swirling ring of hot gas around the black hole—releases this energy primarily in optical and ultraviolet (UV) light. Additionally, a separate region of extremely hot plasma, called the corona, sits above the disk and is responsible for the X-ray emission. The link between these two emissions is well established.
A rare subset of AGNs breaks this pattern, appearing far dimmer in X-rays than expected, often linked to black holes feeding faster than the theoretical limit. This could mean either that gas is blocking the X-rays without dimming the optical/UV, or that it is “intrinsically weak” as the X-ray-emitting corona itself is genuinely suppressed.
‘The curious case of PHL 1811’
For more than two decades, PHL 1811 was the textbook example of an “intrinsically X-ray weak” quasar. At redshift z = 0.192, it is an unusually bright quasar, powered by a black hole with a mass of roughly 180 million times the sun’s mass and feeding at a rate above the theoretical Eddington limit.
Its optical spectrum showed signs of a fast-feeding black hole, and its infrared-to-UV emission looked like that of an entirely ordinary quasar. Yet in X-rays, it was astonishingly faint—almost 180 times fainter than predicted. It did not show signs of gas blocking the X-rays, either.
That changed in 2015, when XMM-Newton/NuSTAR observations found signs of extremely heavy gas blocking most X-rays, with only a small fraction of the emission leaking through.
In August 2024, the Einstein Probe satellite caught PHL 1811 in a bright “flare”—the first time it was ever seen in a normal, unobscured X-ray state. That is exactly what the obscuration model predicts should happen occasionally when the blocking gas moves out of the line of sight.
In this new study, a team of researchers led by Bin Luo of Nanjing University reanalyzed all X-ray data on this object from 2001 to 2024. The team reprocessed every archival X-ray observation of PHL 1811—spanning Chandra, XMM-Newton, Swift and the new Einstein Probe—alongside matching UV measurements to track its X-ray weakness consistently over two decades.
A revisit for clarity
Before 2024, the quasar was extraordinarily faint in X-rays and wildly fluctuating, sometimes shifting threefold within days. Then, during the 2024 flare, its X-ray output suddenly matched normal expectations for the first time before fading again within days.
“We confirmed its extreme X-ray weakness and strong variability across all epochs before 2024, and we found that the 2024 EP flare marks the first and only time that PHL 1811 has been observed in its X-ray nominal state,” the team writes in the paper.
Meanwhile, its optical and infrared brightness barely changed throughout, suggesting the accretion disk stayed stable while something else blocked the X-rays.
This combination of stable optical/UV emission and wildly changing X-rays is difficult to explain if the corona itself is intrinsically weak. Instead, the quasar’s X-rays are usually hidden behind thick, clumpy clouds of gas. “Our results strongly favor the heavy obscuration scenario over the intrinsic X-ray weakness interpretation for PHL 1811,” the researchers conclude.
Researchers note that if PHL 1811 is ever caught significantly brighter than predictions based on the established relation between optical/UV and X-ray emission, that would favor intrinsic coronal variation instead of obscuration. The door is left open for future observations to challenge their conclusion.
For now, the new results suggest that the quasar’s dramatic changes are caused by varying obscuration rather than changes in the black hole itself.
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Publication details
B. Luo et al, The Curious Case of PHL 1811: Heavy Obscuration versus Intrinsic X-Ray Weakness, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae7a5e
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Famous oddball quasar isn’t X-ray weak after all, astronomers say (2026, August 12)
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