Beginning of a giant radio flare from Cygnus X-3

ATel #14821; S. A. Trushkin, A. V. Shevchenko, N. A. Nizhelskij, P. G. (SAO RAS) Tsybulev
on 31 Jul 2021; 12:52 UT
Credential Certification: Sergei Trushkin (satr@sao.ru)

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In the long-term multi-frequency monitoring of the Galactic microquasars with the RATAN-600 radio telescope we have detected a giant flare at 1.2-30 GHz from the microquasar Cyg X-3 (see ATel #13461, #13835). Almost a year after last bright event on 31 July 2021 (MJD 59425.883) we have measured fluxes 0.76, 4.30, 7.73, 7.62, 6.92, 4.43 and 2.86 Jy at 1.24, 2.3, 4.7, 8.2, 11.2, 22.3 and 30 GHz respectively (with errors are less 3-5%) while the binary was in a quenched state about 50 days before (see the figure below). We consider that Cyg X-3 is in the beginning of massive jet ejection that is usually characterized by two regimes: an optically thick one at frequencies lower 4 GHz and by an optically thin one at frequencies higher 5 GHz. The radio spectra during last days are given here . The Swift/BAT data (http://swift.gsfc.nasa.gov/results/transients/CygX-3) show the strong increase of hard X-ray flux at 15-50 keV around MJD 59422. Thus it is a case of a clear association of flaring events in the radio and X-ray bands, related with so-called the disk-jet coupling in the X-ray binary stars as a result of a jet production in the polar regions of accretion disk around a black hole. Again we have met the regular rule: a giant radio flare accurs just after transition from a hyper-soft X-ray state to a soft (later to a hard) state in Cyg X-3 (Koljonen et al., 2010). Following the VLBI mapping, mm-band, X-ray and Gamma-ray observations of Cyg X-3 could be very useful.

RATAN-600 (11GHz) and Swift light curves of the Cyg X-3 in June-July 2021