SVOM/ECLAIRs detection of Cyg X-3 in a likely ultrasoft state
ATel #17585; P.-O. Petrucci (IPAG, France), F. Cangemi (APC, F), A. Coleiro (APC, F), S. Guillot (IRAP, F), L. Tao (IHEP, China), L. Zhang (IHEP, C)
on 8 Jan 2026; 08:58 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)
We report an increase in the X-ray flux of the microquasar Cyg X-3, detected by the ECLAIRs coded-mask instrument onboard the SVOM mission. The source was detected by the Xband quicklook analysis of observations acquired on January 5th, 2026 after 398 s of exposure with a SNR of 47 in the 4-10 keV energy range.
The ECLAIRs long-term lightcurve showed a spectral softening of the source which started in Nov./Dec. 2025. The 4-40 keV spectrum obtained on January 5th, 2026 is well described by a diskbb model (chi2/dof = 11.80/13), with an inner disk temperature of kT = 1.42 +/- 0.07 keV (90% confidence range) and a 4-0 keV flux of (6.3 +/- 0.3)e-9 erg/cm^2/s (90% confidence range). The spectral analysis of the November 4th, 2025 observation shows that the spectrum is well described (chi2/dof = 11.11/9) by an absorbed cutoff power-law model, with a photon index of Gamma = -0.06 +/- 0.74, an energy cutoff of Ecut = 8 (+4) (-2) keV and a hydrogen column density fixed at NH = 8.7e22 cm^2 (Cangemi et al. 2021), yielding a 4-40 keV flux of (5.7 +/- 0.6)e-9 erg/cm^2/s. An iron emission line is also detected in the spectrum, with a centroid energy of 6.4 (+0.3) (-0.8) keV and a line width of 0.4 (+0.8) (-0.3) keV. This clearly indicates that the source underwent a spectral state transition starting from this date and persisting up to the present time.
The current ultrasoft X-ray state is independently confirmed by the MAXI lightcurves (Link).
Cyg X-3 recurrently enters an ultrasoft X-ray state that can last for few weeks, marked by a dominant thermal disk below a few keV and suppressed hard X-rays, often with quenched radio emission. These states, linked to high accretion rates, typically precede major radio flares, connecting disk transitions to powerful jet ejections.
Further multiwavelength follow-up observations are 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.
