Quiet Galaxy Cores Break Silence: Fermi Telescope Captures First Gamma-Ray Glow from Hidden Black Hole Coronae
Astronomers have cracked a three-decade cosmic enigma with the first definitive detection of gamma rays emanating from the superheated coronae surrounding supermassive black holes in "quiet" galaxies. Published in Nature Astronomy July issue, 2025, contributed by a team of international scientists led by Dr. Jian-Min Wang (from Institute of High Energy Physics) and Dr. Jian Li (from University of Science and Technology of China, on behalf of Fermi-LAT collaboration) utilized NASA's Fermi Gamma-ray Space Telescope to observe 37 nearby Seyfert galaxies—radio-quiet active galactic nuclei (AGNs) lacking relativistic jets.
For 30 years, the gamma-ray signature of these black holes remained elusive despite intense scrutiny. The breakthrough came through a sophisticated stacking technique : by combining 15 years of faint gamma-ray signals from individual galaxies, the team amplified the weak emissions into a detectable 5.2-sigma level. The average luminosity measured was (1.5 ± 1.0) × 1040 erg/s in the 1–300 GeV range.
The analysis revealed a startling two-tiered structure:
- A compact corona (∼10 gravitational radii wide, ~1000 light second) producing 1–few GeV gamma rays, matching theoretical predictions of magnetically confined plasma near the black hole.
- An unexpectedly extended corona (2.8 parsecs wide, ∼9 light year) emitting >several GeV gamma rays—defying current models.
To confirm the coronae origin, researchers analyzed a control sample of 27 low-X-ray-flux AGNs and found no gamma-ray detection (TS=2.8). They tested 37 random sky positions and no background contamination is detected. They carefully ruled out other alternatives (star formation, jets, outflows) in AGN via multiwavelength analysis.
The extended structure's immense scale solves a key puzzle: higher-energy gamma rays (> several GeV) should be absorbed via pair production (γγ → e⁺e⁻) in the dense compact corona. The study proposes these gamma rays escape from the diffuse extended region, where X-ray photon density drops with distance squared. The team suggests electron-positron pairs generated in the compact corona expand violently as a "fireball"—akin to gamma-ray bursts—forming the extended corona. During expansion, electrons may be re-accelerated by shocks, sustaining high-energy emission. This dynamic process challenges the long-held view of static coronae.
The extended corona’s discovery forces us to rethink how energy escapes black holes. Upcoming observatories like LHAASO and the Cherenkov Telescope Array could probe these structures at TeV energies, potentially linking them to high-energy cosmic rays and neutrinos. This hidden gamma-ray glow—invisible to individual galaxy observations—was finally unveiled through patient data archaeology. The findings validate decades of theoretical work while exposing new complexities in black hole physics.
Access the study:
Fermi detection of gamma-ray emission from the hot coronae of radio-quiet active galactic nuclei Nature Astronomy (2025)