Fermi detection of gamma-ray Emission from the Hot Coronae of Radio-quiet Active Galactic Nuclei

Abstract

Relativistic jets around supermassive black holes (SMBHs) are well-known powerful ${\hat{I}}^3$-ray emitters. In absence of the jets in radio-quiet active galactic nuclei (AGNs), how the SMBHs work in ${\hat{I}}^3$-ray bands is still unknown despite of great observational efforts made in the last 3 decades. Considering the previous efforts, we carefully select an AGN sample composed of 37 nearby Seyfert galaxies with ultra-hard X-rays for the goals of ${\hat{I}}^3$-ray detections by excluding all potential contamination in this band. Adopting a stacking technique, here we report the significant ${\hat{I}}^3$-ray detection ($\mathrm{TS}=30.6$, or $5.2\ddot{I}ƒ$) from the sample using 15-year Fermi-Large Area Telescope (LAT) observation. We find an average ${\hat{I}}^3$-ray luminosity of the sample as $(1.5\pm 1.0)\times 10^{40}\mathrm{erg}{\mathrm{s}}^{-1}$ at energies from 1-300\,GeV. Limited by the well-known pair production from the interaction of ${\hat{I}}^3$-rays with low energy photons, $\gtrsim$ several GeV ${\hat{I}}^3$-rays are found to originate from an extended corona ($\sim 2.7\times 10^6{R}_{\mathrm{g}}$), whereas the canonical much more compact X-ray corona ($\sim 10R_{\mathrm{g}}$) is responsible for 1 to several GeV ${\hat{I}}^3$-rays. The finding of the compact region lends to strong supports to the long-time theoretical expectations, but the extended corona is beyond all the existing models. One promising scenario is that the electron-positron pairs produced in the compact X-ray corona would expand as fireball, similar to that in ${\hat{I}}^3$-ray bursts, forming the structure of extended corona.

Published in Nature Astronomy