Famous oddball quasar isn't X-ray weak after all, astronomers say
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…
For over two decades, the quasar PHL 1811 has been viewed as the prototypical example of an intrinsically X-ray weak quasar. Astronomers assumed it produced significantly less X-ray radiation than expected due to a fast-feeding black hole. However, in 2024, the Einstein Probe spacecraft observed PHL 1811 in a bright X-ray flare — the first time it was seen in a normal, unobscured state in over two decades.
A new study led by Bin Luo of Nanjing University combined this 2024 observation with more than 20 years of archival data to reassess the quasar's X-ray behavior. The researchers analyzed every X-ray observation of PHL 1811 from 2001 to 2024, alongside matching ultraviolet measurements. Before 2024, PHL 1811 was exceptionally faint in X-rays and displayed highly variable X-ray output, sometimes fluctuating by a factor of three within days.
In August 2024, the quasar suddenly brightened to normal levels, only to fade again within days. This dramatic change in the quasar's X-ray behavior, while its optical and infrared brightness remained stable, suggests that the variability is caused by heavy, clumpy gas clouds obscuring the X-rays, rather than an intrinsic weakness in the quasar's X-ray-emitting corona.
The team concludes that their findings strongly support the heavy obscuration scenario over the idea that PHL 1811 is intrinsically weak in X-rays.
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