Exceptional gamma-ray flaring activity of the blazar S4 0954+65 in early 2025
Year: 2026
Type: article
Abstract: Context. S4 0954+65 (4FGL J0958.7+6534) is a TeV-detected blazar at a redshift of z = 0.3694 ± 0.0011, classified as an intermediate-synchrotron-peaked BL Lac object. In early 2025, it entered an exceptional γ -ray high state, exceeding the flux of the 2015 outburst, during which contemporaneous MAGIC TeV observations were obtained. This event offers an opportunity to investigate the radiation mechanisms, particle acceleration, and jet dynamics. Aims. 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. Methods. We performed a multiwavelength analysis using γ -ray, X-ray, optical/UV, radio, and 43 GHz Very Long Baseline Array (VLBA) observations. We examined multiband 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). Results. 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 anticorrelated with the 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. Conclusions. 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 multiwavelength observations with physically plausible parameters. A lepto-hadronic interpretation remains possible but requires substantially higher jet power.
Source: Astronomy and Astrophysics
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Northwestern University, Drexel University, European Space Agency, University of Leicester, University of Warwick, National Natural Science Foundation of China, Chinese Academy of Sciences, Shanghai Education Development Foundation, Shanghai Municipal Education Commission, Stockholms Universitet, National Key Research and Development Program of China, Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning
OpenAlex ID: W7214354481