Spectral hardening of Cygnus X-3 observed with SVOM/ECLAIRs

ATel #18049; F. Cangemi (APC, France), S. Le Stum (APC, F), S. Guillot (IRAP, F), A. Coleiro (APC, F), L. Tao (IHEP, China), L. Zhang (IHEP, C)
on 16 Sep 2026; 08:02 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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We report the detection of a spectral hardening of the microquasar Cygnus X-3 with the ECLAIRs coded-mask telescope onboard the SVOM mission on 2026 September 13.

The long-term ECLAIRs light curve shows a progressive hardening of the source between 2026 August 30 and September 13, the latter being the most recent observation of Cygnus X-3 within the ECLAIRs field of view. Over this period, the hardness ratio (defined as the 10-20 keV count rate divided by the 4-10 keV count rate) increased from 0.06 +/- 0.01 on August 30 to 0.38 +/- 0.01 on September 13.

A preliminary spectral analysis of the ECLAIRs 4-100 keV spectrum obtained on September 13 (exposure time of 2547 s) shows that the spectrum is well described (chi2/dof = 29.67/23) by a cutoff power-law model, with a photon index of Gamma = 1.9 (+/- 0.3) and a cutoff energy of E_cut = 27 (+25 / -10) keV. An additional iron emission line is detected and is modelled with a Gaussian centered at 6.5 +/- 0.2 keV, with its width fixed at 0.4 keV. The corresponding unabsorbed 4-10 keV and 10-50 keV fluxes are (3.53 +0.07 / -0.43)e-9 and (3.25 +0.09 / -1.67)e-9 erg/cm2/s, respectively. All uncertainties are given at the 90% confidence level.

Cygnus X-3 was observed by SVOM/ECLAIRs in a very soft state on May 27 (ATel #17820), followed by major radio flaring activity (ATels #17836, #18001). The observations reported here from September 13 indicate that Cygnus X-3 is evolving toward a harder spectral state, as generally expected following strong radio activity.

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.