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We present X-ray spectral and timing behavior of Cyg X-3 as observed byAstroSat during the onset of a giant radio flare on 01-02 April 2017. Within atime-scale of few hours, the source shows a transition from the hypersoft state(HPS) to a more luminous state (we termed as the very high state) whichcoincides with the time of the steep rise in radio flux density by an order ofmagnitude. Modeling the SXT and LAXPC spectra jointly in 0.5-70.0 keV, we foundthat the first few hours of the observation is dominated by the HPS with nosignificant counts above 17 keV. Later, an additional flat powerlaw componentsuddenly appeared in the spectra which extends to very high energies with thepowerlaw photon index of 1.49 +/- 0.04. Such a flat powerlaw component hasnever been reported from Cyg X-3. Interestingly the fitted powerlaw model in25-70 keV, when extrapolated to the radio frequency, predicts the radio fluxdensity consistent with the trend measured from RATAN-600 telescope at 11.2GHz. This provides a direct evidence of the synchrotron origin of flat X-raypowerlaw component and the most extensive monitoring of the broadband X-raybehavior at the moment of decoupling the giant radio jet base from the compactobject in Cyg X-3. Using SXT and LAXPC observations, we determine the giantflare ejection time as MJD 57845.34 +/- 0.08 when 11.2 GHz radio flux densityincreases from ~100 to ~478 mJy.

Extensive broadband X-ray monitoring during the formation of a giant radio jet base in Cyg X-3 with AstroSat