Exceptional Gamma-Ray Flaring Activity of the Blazar S4 0954+65 in Early 2025

Abstract

S4 0954+65 (4FGL J0958.7+6534) is a TeV-detected blazar at a redshift of $z = 0.3694 \pm 0.0011$, classified as an intermediate-synchrotron-peaked BL Lac object. In early 2025, it entered an exceptional $γ$-ray high state. We aim to investigate the origin and physical properties of the exceptional 2025 flare, and to constrain the emission processes responsible for this flaring activity. We performed a multi-wavelength analysis using $γ$-ray, X-ray, optical/UV, radio, and 43 GHz Very Long Baseline Array (VLBA) observations. We examined multi-band correlations with the z-transformed discrete correlation function (zDCF), analyzed the spectral evolution and parsec-scale jet kinematics, and modeled the broadband spectral energy distributions (SEDs). The $γ$-ray variations lead the optical and radio emission by 3.22 days and possibly 18.26 days, respectively, while no significant correlation is found between the $γ$-ray and X-ray emission. The $γ$-ray spectra show a harder-when-brighter behavior, consistent with enhanced particle acceleration during the active state. A similar spectral trend is also observed in the X-ray band, where the photon index is anti-correlated with flux. During the major $γ$-ray flare, VLBA images reveal the emergence of a new superluminal radio knot from the core, whose extrapolated ejection time is consistent with the peak of the $γ$-ray flare. The temporal and structural evolution during the 2025 flare is consistent with a shock-in-jet scenario, in which a newly emerging disturbance propagates downstream and drives the flare. Broadband SED modeling shows that a leptonic synchrotron self-Compton plus external Compton model with dusty torus seed photons reproduces the multi-wavelength observations with physically plausible parameters. A lepto-hadronic interpretation remains possible but requires substantially higher jet power.