The First GeV γ -Ray Flares from the Compact Symmetric Object–like Source 4C 76.03

Abstract We report the first detection of GeV γ -ray flaring activity from the compact symmetric object (CSO)–like source 4C 76.03, based on 17 yr of Fermi-Large Area Telescope (LAT) observations. Its long-term, time-averaged γ -ray properties are consistent with the 4FGL-DR4 catalog. However, a time-resolved analysis with 100 day binning reveals two prominent flares occurring on timescales of approximately 30 and 20 days, separated by ∼2.5 yr, with nearly identical fluxes, test statistic (TS) values, and photon indices. The short-timescale variability indicates localized and transient energy dissipation in the nuclear region, likely associated with newly injected jet components. Although the γ -ray emission does not directly trace the long-term jet power responsible for building the observed radio structure, it demonstrates that the central engine remains active. In the context of CSO evolution, 4C 76.03 may represent a rare transitional case, where repeated energy injections allow the source to exceed the canonical ∼500 pc scale of most CSOs, providing key insight into the early stages of radio jet evolution.