In the vast cosmic theater, supermassive black holes are among the most enigmatic and powerful actors. Their dramatic behavior is often witnessed through spectacular jets that spew high-energy particles and radiation across the universe, prominently including gamma rays—the most energetic form of light. These relativistic jets, commonly found in radio-loud active galactic nuclei (AGN), have been extensively studied and are well-established sources of gamma-ray emissions. Yet, an intriguing puzzle persists: how do radio-quiet AGNs, which lack these powerful jets, contribute to the gamma-ray sky? Despite decades of observational efforts, the mechanisms underpinning gamma-ray production in such systems remained elusive—until now.
A groundbreaking study, recently published by Liu, Wang, and the Fermi-LAT Collaboration, has opened a new window into this mystery. By meticulously analyzing a carefully curated sample of 37 nearby Seyfert galaxies—classic examples of radio-quiet AGNs characterized by their intense ultrahard X-ray emissions—the researchers report a significant detection of gamma rays originating from these enigmatic sources. Using an innovative stacking technique applied to fifteen years of data from the Fermi Large Area Telescope (Fermi-LAT), the team uncovered compelling evidence that challenges previous assumptions about the gamma-ray environment surrounding supermassive black holes in radio-quiet galaxies.
The Fermi-LAT, a space-based observatory launched in 2008, has revolutionized our understanding of the gamma-ray universe. It continuously surveys the sky, capturing photons with energies ranging from millions to billions of electronvolts. However, detecting faint gamma-ray signals from individual radio-quiet AGNs amidst the noisy cosmic backdrop has proven exceptionally challenging. The researchers’ approach was to select a sample of Seyfert galaxies with the least amount of contamination from background sources and then combine their gamma-ray signals to enhance the overall detectability. This stacking method leveraged the accumulated photons over more than a decade, boosting the sensitivity to levels capable of revealing the subtle gamma-ray glow of these otherwise inconspicuous galactic nuclei.
The analysis yielded a strikingly significant result: the detection of gamma rays with a test statistic of 30.6, corresponding to a statistical significance of 5.2 sigma—a threshold commonly regarded as solid proof in astrophysical observations. Intriguingly, the average gamma-ray luminosity of the sample was measured to be about 1.5 × 10^40 erg per second in the 1–300 GeV (giga-electronvolt) energy range. This luminosity, while modest compared to the luminous jets in radio-loud AGNs, is substantial enough to confirm the presence of high-energy processes at play in the nuclei of these galaxies.
Understanding the origin of these gamma rays requires a deep dive into the complex environment near the central black hole. Traditionally, it has been thought that gamma rays in AGNs emanate predominantly from jet structures, but in Seyfert galaxies, there are no powerful jets to account for such emission. Instead, the researchers turned their focus to the hot corona—a compact, energetic region of electrons enveloping the black hole’s accretion disk, known for producing strong X-ray radiation. This corona has long been theorized to accelerate particles to relativistic speeds, potentially generating gamma rays, but direct evidence had been scarce.
Surprisingly, the study reveals a bifurcated source of gamma-ray production correlated with the energy scale of the photons. Photons in the range of one to several giga-electronvolts appear to originate from a canonical, compact corona on the scale of about ten gravitational radii—a unit defined by the black hole’s mass. This finding aligns well with existing theoretical models which depict the corona as a dense, hot plasma close to the event horizon, where intense magnetic fields and particle acceleration can yield gamma-ray photons.
More unexpectedly, the gamma rays with energies exceeding several giga-electronvolts point to a vastly larger emission region. The data indicate an extended corona stretching approximately 2.7 million gravitational radii from the black hole. This scale dwarfs the traditional corona concepts and suggests an extended, diffusely emitting structure enveloping the central engine of the AGN. The existence of this extended corona challenges existing paradigms and demands a reevaluation of the physical processes governing gamma-ray production in radio-quiet AGNs.
A plausible interpretation proposed by the researchers involves the formation of a fireball of electron-positron pairs. These pairs are generated in the intense magnetic and radiation fields of the compact X-ray corona through high-energy photon interactions. Instead of remaining confined, these pairs could expand outward, inflating to form a large-scale, tenuous corona resembling the fireball structures observed in gamma-ray bursts. This expanding plasma could naturally explain the extended origin of the highest-energy gamma rays detected.
This discovery not only illuminates the dynamic behavior of supermassive black holes without jets but also extends our grasp of particle acceleration and radiation mechanisms in extreme gravitational fields. The insight that Seyfert galaxies can produce significant gamma-ray emission via their hot coronae expands the population of known gamma-ray emitters and suggests these ubiquitous galaxies contribute meaningfully to the extragalactic gamma-ray background.
Moreover, the detection sheds light on the interplay between the dense photon fields near the black hole and the processes that limit gamma-ray escape. Gamma rays traversing the environment risk annihilation through pair production, where a gamma-ray photon interacts with a lower-energy photon to produce an electron and a positron. This interaction naturally imposes energy-dependent spatial constraints on where gamma rays can be produced and subsequently escape—consistent with the compact and extended corona dichotomy observed.
The implications of this research extend beyond astrophysics, touching on fundamental physics. The extreme conditions inferred—magnetic field strengths, particle densities, and relativistic dynamics—provide a natural laboratory for testing high-energy processes and plasma physics under regimes unattainable on Earth. Furthermore, they help refine models of black hole growth, feedback, and the cosmic ecosystem shaping galaxy evolution.
Complementary multiwavelength observations can now target these Seyfert galaxies to further unravel the structure and dynamics of their hot coronae. Future gamma-ray facilities, combined with X-ray and radio measurements, will test the fireball scenario and explore whether similar mechanisms are ubiquitous in less active or dormant galactic centers. The extended corona hypothesis, if confirmed, could revolutionize our understanding of how black holes influence their surroundings even in the absence of conspicuous jets.
This landmark study by Liu, Wang, and colleagues underscores the power of long-term, sensitive gamma-ray observations combined with meticulous source selection and innovative analysis techniques. It redefines the role of radio-quiet AGNs as gamma-ray sources and opens new theoretical frontiers to explain the complex, multi-scale structures around supermassive black holes. By bridging the gap between high-energy astrophysics and black hole physics, this work invites a reconsideration of the energetic interplay governing the fate of matter and radiation in some of the universe’s most extreme environments.
As the cosmic narrative unfolds, we stand on the cusp of uncovering how seemingly quiet galactic nuclei transform the dark abyss around their supermassive black holes into luminous beacons of the high-energy universe. The detection of gamma-ray emission from hot coronae paints a richer, more intricate portrait of black hole ecosystems, deepening our quest to comprehend the fundamental workings of the cosmos.
Subject of Research: Gamma-ray emission mechanisms in radio-quiet active galactic nuclei, specifically focusing on hot corona structures around supermassive black holes.
Article Title: Fermi detection of gamma-ray emission from the hot coronae of radio-quiet active galactic nuclei.
Article References:
Liu, JR., Wang, JM. & Fermi-LAT Collaboration. Fermi detection of gamma-ray emission from the hot coronae of radio-quiet active galactic nuclei. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02538-2
Image Credits: AI Generated
