Extraordinary radio flaring activity of X-ray binary Cyg X-3 in 2026

ATel #17836; S. A. Trushkin, P. G. Tsybulev, N. N. Bursov, (SAO RAS, Russia)
on 7 Jun 2026; 09:42 UT
Credential Certification: Sergei Trushkin (satr@sao.ru)

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We carry out the multi-frequency monitoring of the Galactic microquasars with the RATAN-600 radio telescope on 10 January, 6 and 26 April, and now on 6 June 2026 we detected the giant flares from the X-ray and Gamma-ray binary, microquasar Cygnus X-3 at frequencies 1.45 - 30 GHz (see ATel #17742). The three first flares were brighter than 10 Jy at 4.7 GHz, and higher that 20 Jy at 1.45 GHz and probably current flare in June will be bright also. The flares are characterized by the optically thin synchrotron spectra with power law dependence at high frequencies with variable spectral indices (from -0.3 to -0.8) and optically thick spectra with positive spectral indices (usually from +1 to +2) at lower frequencies in first 1-3 days. All these flares associated with the X-ray transition from long-term hyper-soft X-ray states to hard states according Swift/BAT data (https://swift.gsfc.nasa.gov/results/transients/CygX-3). Moreover the flares are clearly detected in Gamma-rays (0.1-300 GeV) with Fermi-LAT observatory (https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/Cygnus_X-3). Spectra of Cyg X-3 on 6 and 7 June (and will be further), are given here . The light curves at 2.25, 4.7 and 8.2 GHz and Fermi/LAT fluxes are given below. Multi-band observations are encouraged as soon as possible.

Radio light curves of Cyg X-3 from 5 April 2026 and Fermi/LAT fluxes at 0.1-300 GeV