Unexpected Gamma-ray Outburst from the CSS Radio Galaxy 3C 216

Compact steep-spectrum (CSS) radio galaxies are generally young or intermittent/frustrated-jet systems trapped in gas-rich environments, and they are rarely detected in GeV gamma-ray bands. 3C 216 is an interesting exception among this class.

In 2023, the Large Area Telescope (LAT) on board NASA's Fermi Gamma-ray Space Telescope detected an unexpected, rapid, and powerful GeV gamma-ray outburst from 3C 216. The outburst was followed up with ultraviolet and X-ray observations by the Neil Gehrels Swift Space Observatory.

During the outburst, 3C 216's spectral energy distribution was dominated by synchrotron and synchrotron self-Compton radiation produced in the relativistic jet, suggesting a young quasar classification. Radio-band features—superluminal motion and a flat-spectrum core component—confirm that 3C 216 harbors a distinct blazar-like core that is bent and viewed at a small angle to the line of sight. This is the unique large and energetic event from the source observed in 17 years of Fermi-LAT continuous all-sky and time-domain monitoring.