SVOM/ECLAIRs detection of the blazar 1ES 1959+650

ATel #18077; F. Cangemi (APC, France), S. Le Stum (APC, F), A. Coleiro (APC, F), P.-O. Petrucci (IPAG, F), L. Tao (IHEP, China), L. Zhang (IHEP, C)
on 23 Sep 2026; 15:48 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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We report the detection of the blazar 1ES 1959+650 with the ECLAIRs telescope onboard the SVOM mission on September 18, between 14:26:39.898 and 17:12:50.386 UTC, with a signal-to-noise ratio of 15.79 in the 4-20 keV energy band, while the source was within the ECLAIRs field of view. This detection occurred shortly before the detection at VHE reported in ATel #18065.

A preliminary spectral analysis with ECLAIRs (exposure of 6854 s) data shows that the 4-50 keV spectrum is well fitted (fit statistic = 12.36/9 dof) by a powerlaw model, with a photon index of Gamma = (2.3 +0.3 / -0.2). The corresponding 4-50 keV observed flux is (5.3 +1.5 / -0.6) e-10 erg/cm2/s. In addition, the source was also within the ECLAIRs field of view on September 20 at 05:01:38.026 UTC, for an exposure time of 1293 s. The 4-50 keV spectrum can be well fitted (fit statistic = 10.41/9 dof) with a powerlaw, with a photon index of Gamma = (1.9 +/- 0.2). We observe an increase in the observed 4-50 keV flux compared to the September 18 detection, reaching (1.2 +0.2 / -0.1) e-9 erg/cm2/s. All errors are at the 90% confidence level.

Following these detections, a SVOM Target of Opportunity observation was performed on September 22 to further investigate the ongoing activity of the source.

1ES 1959+650 is a high-synchrotron-peaked BL Lac object, with its synchrotron peak located in the ultraviolet, and exhibits recurrent optical activity. Its last reported flaring activity occurred in December 2024 (ATels #16935, #16939, #16941, #16955, #16959, #16978).

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.