Astronomers have tracked a dramatic “changing-look” active galactic nucleus (AGN) whose central supermassive black hole appeared to switch off and then rapidly reignite. The galaxy, HE 1237−2252, dimmed in X-rays by a factor of 17 within just 18 months before recovering again. The paper outlining its analysis was uploaded to the arXiv preprint server on May 8.
On and off
AGNs are powered by matter spiraling into a supermassive black hole through an accretion disk. These black holes undergo different feeding episodes that may end up turning them off or on, altering their luminosity over timescales of tens of thousands of years. In recent years, astronomers have been detecting AGNs that undergo major changes in their feeding activity on timescales of months to years. These shifts cause the object to transition between optical spectral types.
All AGNs are classified into two broad types based on the kind of emission lines in their spectra. Type 1 exhibits both broad and narrow emission lines, indicative of the gas being much closer and far from the black hole, respectively. Type 2 galaxies show only the narrow emission lines.
In between sit intermediate subtypes—1.2, 1.5, 1.8, and 1.9—ranked by how strong the broad emission is relative to the narrow. For example, a Type 1.8 shows one broad Hα line clearly and broad Hβ weakly; a Type 1.9 shows just a broad Hα line; and a Type 2 shows no broad components at all. When a galaxy shifts between these types, astronomers call it a “changing-look AGN” (CLAGN).
The reason these objects change their “look” is not fully understood. Plausible explanations include a genuine drop in the black hole’s feeding rate, local instabilities in the accretion disk, or external disruptions. There is also debate about whether the broad-line region is a permanent gas reservoir or a wind that only exists when the black hole is feeding above a certain threshold.
Caught in the act
Most changing-look AGNs have been identified through repeat optical spectroscopy or large-area photometric surveys, with over 150 confirmed to date. In January 2022, using the Extended ROentgen Survey with an Imaging Telescope Array (eROSITA), astronomers, led by Alex Markowitz of the Nicolaus Copernicus Astronomical Center, identified a Seyfert galaxy—a class of AGN with extremely bright nuclei, hosted in spiral galaxies whose structure remains clearly visible, unlike quasars—whose soft X-ray flux had collapsed by a factor of approximately 17 within just 18 months.
This Seyfert galaxy, cataloged as J1240–2309 or HE 1237–2252, sits roughly 1.3 billion light-years away. Initially classified as Type 1.0–1.2, it was found to be Type 1.8 at the time of observation in 2022. Data across multiple wavelengths showed the dimming was not limited to X-rays—infrared brightness had also dipped—and the team’s follow-up multiwavelength campaign from late 2022 through early 2025 tracked how each band slowly recovered. The X-ray flux recovered within just three months, but optical, UV, and infrared bands recovered more slowly, over roughly three years.
By late 2024, the galaxy had returned to Type 1.0, with its spectral lines fully recovered. As the black hole reignited, the researchers also detected a distinct double-peaked pattern in the hydrogen emission lines, hinting at a ring-like structure of gas near the disk being lit up by the recovering corona.
The bigger picture
If a passing dust cloud had simply drifted across our line of sight to the black hole, dimming its luminosity, two things would not have happened: the X-ray data would have shown telltale signs of absorption, and the infrared brightness would have stayed steady. Since both dropped significantly, researchers proposed that the black hole was producing less energy, pointing to an intrinsic change within the accretion disk itself.
The leading explanation involves “cold” and “warm” fronts propagating through the accretion disk—a cooling wave temporarily suppressing the disk’s output, followed by a warming wave that gradually restores it.
Researchers say more observation time and more examples of such systems are needed to get the complete picture of the changing-look AGN. “If such fronts are relevant for explaining the behavior of other CLAGN, then we encourage future optical/UV monitoring campaigns to adopt a high cadence to better test and constrain this class of models,” the researchers write.
This study offers one of the clearest real-time views yet of a black hole dramatically changing its feeding rate and slowly coming back to life.
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Publication details
Alex Markowitz et al, A Changing-Look Seyfert Discovered by eROSITA Reveals a Two-Component Broad-Line Region, arXiv (2026). DOI: 10.48550/arxiv.2605.07965
Journal information:
arXiv
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eROSITA discovers a ‘changing-look’ Seyfert galaxy (2026, May 17)
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