Fermi detection of gamma-ray emission from the hot coronae of radio-quiet active galactic nuclei

Abstract

Relativistic jets around supermassive black holes are well-known powerful γ-ray emitters. In the absence of the jets in radio-quiet active galactic nuclei, how the supermassive black holes work in γ-ray bands is still unknown despite great observational efforts in the past three decades. Here, considering the previous efforts, we carefully select an active galactic nucleus sample composed of 37 nearby Seyfert galaxies with ultrahard X-rays for γ-ray detection by excluding all potential contamination in this band. Adopting a stacking technique, we report the significant γ-ray detection (test statistic 30.6, or 5.2σ) from the sample using 15-year Fermi-LAT observations. We find the average γ-ray luminosity of the sample to be (1.5 ± 1.0) × 10⁴⁰ erg s⁻¹ at energies of 1–300 GeV. Limited by the well-known pair production from the interaction of γ-rays with low-energy photons, γ-rays of more than several giga-electronvolts are found to originate from an extended corona (~2.7 × 10⁶ gravitational radii), whereas the canonical much more compact X-ray corona (~10 gravitational radii) is responsible for γ-rays of one to several giga-electronvolts. The finding of the compact region lends strong support to the long-time theoretical expectations, but the extended corona is an unexpected finding. One promising scenario is that the electron–positron pairs produced in the compact X-ray corona would expand as a fireball, similar to that in γ-ray bursts, forming the structure of extended corona.