SVOM discovery of a strong X-ray outburst of the blazar 1ES 1959+650 and multiwavelength follow-up with the Neil Gehrels Swift observatory

Context. On December 6, 2024, 1ES 1959+650, one of the X-ray brightest blazars known, underwent a high-amplitude X-ray outburst detected by SVOM, the first such discovery with this mission. The source was subsequently monitored with SVOM and Swift from December 2024 to March 2025. Aims. We report the detection and multiwavelength follow-up of this event, and describe the temporal and spectral evolution observed during the campaign. Methods. We analyzed data from SVOM/MXT, SVOM/ECLAIRs, and Swift/XRT using log-parabola models to track flux and spectral variability. Results. We detected the source in a bright state over the 0.3–50 keV range. During the three months of monitoring, the X-ray flux varied significantly, and showed episodes of spectral hardening at high flux levels. The spectral curvature evolved more irregularly and did not show a clear trend with flux. The synchrotron peak of the spectral energy distribution shifts to higher energies when the flux increases. Conclusions. This event constitutes the first blazar outburst discovered in X-rays by SVOM. The coordinated follow-up with Swift provided continuous coverage of the flare and highlights the strong complementarity of the two missions for time-domain studies of blazars. The flare shows no clear signatures of either Fermi I or Fermi II acceleration, suggesting a mixed Fermi I and II scenario.