The beginning of a giant radio flare of Cygnus X-3

ATel #17600; S. A. Trushkin, N. N. Bursov, P. G. Tsybulev, A. V. Shevchenko (SAO RAS, Russia)
on 15 Jan 2026; 15:55 UT
Credential Certification: Sergei Trushkin (satr@sao.ru)

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During the multi-frequency monitoring of the Galactic microquasars with the RATAN-600 radio telescope we detected the giant flare from the microquasar Cygnus X-3 on 15 January 2026 at frequencies 1.42-30 GHz. Cyg X-3 entered in so-called hyper-soft X-ray state 40 day ago, when hard X-ray emission dropped to almost zero (Swift/BAT), showing high soft X-ray (MAXI) fluxes. Then the microquasar showed some precursor small flares (~1 Jy) and just now it is flaring. Today (MJD 61055.421) the flux increased up to 6.1 Jy at 4.7 GHz. The spectrum evolved from the spectral index about 0.0 (S_nu~nu^alpha) (on 14 Jan) to optically thin spectrum with alpha = -0.6 at higher frequencies. Thus the flux have increased from 24 mJy on 8 Jan to 6.1 Jy at 4.7 GHz today. The significant (5-sigma) Gamma-ray flux of 1.463322E-06 photons/cm^2/s at 0.1-300 GeV was detected by Fermi observatory on 10 Jan 2026 (MJD 61050.5). Before we have detected the last giant flare (9.5 Jy at 2.3 GHz) in August 2024 (MJD 60544), and followed quiescence state stayed around the level of 50-200 mJy up to the current activity. See spectra during the current flare here .

Swift/BAT and 4.7 GHz light curves of Cyg X-3 during last 200 days