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

ATel #17820; S. Le Stum (APC, F), F. Cangemi (APC, France), A. Coleiro (APC, F), S. Guillot (IRAP, F), L. Zhang (IHEP, China)
on 29 May 2026; 07:12 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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We report a spectral softening of the microquasar Cygnus X-3 detected on 2026 May 27 by the ECLAIRs coded-mask instrument onboard the SVOM mission.

The long-term ECLAIRs light curve had shown a progressive hardening of the source from 2026 March 30 until 2026 May 12. When the source was again in the ECLAIRs field of view on 2026 May 27, we detected a significant decrease in the flux above 10 keV. In particular, the count rate in the 10-20 keV band dropped to 4.46 +/- 0.44 c/s on May 27, compared to 28.75 +/- 0.97 c/s measured on May 12 in the same energy band.

A preliminary spectral analysis of the 2026 May 27 ECLAIRs 4-50 keV spectrum (exposure time of 1396 s) shows that the spectrum is very soft and is well fitted (chi2/dof = 32.12/26) by a disk blackbody model (at a temperature kT = 1.31 +/- 0.03 keV) and an iron emission line fitted with a Gaussian centered at 6.56 +/- 0.08 keV. The 10-50 keV flux is 2.4 (+/- 0.2) e-10 erg/s/cm2. In comparison, the 4-50 keV spectrum obtained on 2026 May 12 (exposure time of 4317 s) is well fitted (chi2/dof = 37.64/23) by a cutoff power-law model with a photon index of gamma = 2.4 +/- 0.6 and a cutoff energy of Ecut = 14 (+57 / -7) keV. An iron line is also detected at 6.4 +/- 0.1 keV. The corresponding 10-50 keV flux is 2.1 (+0.1 /-0.9) e-09 erg/s/cm2. All uncertainties are given at the 90% confidence level. These results indicate that the source transitioned to a significantly softer spectral state between 2026 May 12 and May 27.

A fading of the radio flux was recently reported on May 22, 23, and 24 (ATel #17814) and is consistent with the suppression of the hard X-ray emission reported here. The X-ray spectral analysis of the 2026 May 27 data suggests that the source is likely in an ultrasoft state, characterized by a dominant thermal disk component. Such ultrasoft states are often observed prior to major radio flares, and monitoring of this source is 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.