SVOM/ECLAIRs Detection of the Current Exceptional Very-High Energy Flare from Mrk 421
ATel #17595; S. Le Stum (APC, France), F. Cangemi (APC, F), A. Coleiro (APC, F), N. Dagoneau (CEA, F), L. Tao (IHEP, China), L. Zhang (IHEP, C), S. Zheng (IHEP, C)
on 14 Jan 2026; 14:40 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)
Referred to by ATel #: 17622
We report the detection of an X-ray flare from the blazar Mrk 421, observed with the 4-150 keV ECLAIRs coded-mask telescope onboard the SVOM satellite. The source has been regularly located within the ECLAIRs field of view since December 23rd 2025. A significant increase in the X-ray flux has been observed in the ECLAIRs Quick Look Analysis, starting around January 6th 2026. The count rate reached a peak of 27.0 +/- 0.5 counts per second in the 4-10 keV energy range on January 11th, compared to a typical count rate of 6.0 +/- 0.5 counts per second prior to January 6th in the same energy band. This represents the highest count rate of the source detected by ECLAIRs since the launch of SVOM in June 2024.
A preliminary spectral analysis indicates that the January 11th spectrum in the 4-100 keV band (exposure time of 8435 s) is well fitted (chi2/dof = 17.81/12) by a power-law model. The best-fit photon index is Gamma = 2.12 +/- 0.07, and the energy flux is (3.3 +/- 0.2) e-9 ergs/cm2/s in the 4-100 keV range. No significant variation in the photon index is observed in the spectra obtained on January 6th (exposure time of 5457 s) and January 13th (exposure time of 4222 s). However, the corresponding 4-100 keV fluxes are 8.7 (+1.2, -1.7) e-10 and 8.3 (+1.2, -3.8) e-10 ergs/cm2/s, respectively, indicating a decrease in flux after January 11th.
Extrapolating the energy flux to the 0.3-10 keV energy band, it reaches approximately 4.8e-9 ergs/cm2/s, making this event possibly the brightest X-ray flare observed since December 2017 (Kapanadze et al. 2018).
This X-ray activity is temporally correlated with the exceptional gamma-ray flare reported by VERITAS on January 11th, 2026 (ATel #17594). Further multi-wavelength observations and analysis are strongly encouraged.
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.
