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

¹ Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
² Max-Planck-Institut für Radiostronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Department of Physics, Geology, and Engineering Technology, Northern Kentucky University, 1 Nunn Drive, Highland Heights, KY 41099, USA
⁴ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, 11 rue de l’Université, F-67000 Strasbourg, France
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁷ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁸ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁹ IRAP, CNRS, 9 avenue du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France
¹⁰ CEA Paris-Saclay, Irfu / Département d’Astrophysique, F-91191 Gif-sur-Yvette, France
¹¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
¹² School of Astronomy and Space Science, Nanjing University, 210023 Nanjing, Jiangsu, China
¹³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing, PR China
¹⁴ School of Physics, Beijing Institute of Technology, Beijing 100081, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 November 2025
Accepted: 9 January 2026

Abstract

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.