A broadband outburst of the compact steep-spectrum quasar 3C 138 in 2024-2026
After several decades of relative quiescence, the compact steep-spectrum quasar 3C 138 entered an active phase in 2024-2026, exhibiting strong broadband flaring. We investigate its multiwavelength behaviour using dense multifrequency radio monitoring at 1-22 GHz with RATAN-600 and RT-32, optical R-band observations with Zeiss-1000 and AS-500/2, X-ray measurements with Swift/XRT and SRG/ART-XC, and the Fermi-LAT $γ$-ray light curve. The radio brightening accelerated after 2022 and was strongest at the highest frequencies. The radio spectra hardened markedly, with the 11-22 GHz spectral index evolving from steep to flat or inverted during the active phase. The X-ray flux increased by more than a factor of three during 2025-2026, while the photon index hardened from $Γ_{\rm X}\simeq 1.6$ to $Γ_{\rm X}\simeq 0.9$ and softened back after the peak. Flare decomposition revealed five $γ$-ray flares and a sequence of optical subflares during the later stages of the activity. The $γ$-ray, X-ray, and optical maxima occur within a $\simeq 13$-day interval, suggesting a common activity episode, whereas the radio brightens more gradually and in a frequency-dependent manner. Under the adopted compact-zone geometries, the sparse two-state spectral energy distributions (SEDs) can be represented by one-zone synchrotron self-Compton (SSC) solutions, while the relative contribution of external Compton (EC) remains geometry dependent. The flare shifts the modelled energy partition towards relativistic electrons. These results favour a longer-lived, core-dominated activity phase, with later high-energy and optical flares superposed on the opacity-driven radio evolution of an emerging synchrotron component.