We present X-ray spectral and timing behavior of Cyg X-3 as observed by AstroSat during the onset of a giant radio flare on 01-02 April 2017 .
Within a time-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 ) which coincides with the time of the steep rise in radio flux density by an order of magnitude .
Modeling the SXT and LAXPC spectra jointly in 0.5-70.0 keV , we found that the first few hours of the observation is dominated by the HPS with no significant counts above 17 keV .
Later , an additional flat powerlaw component suddenly appeared in the spectra which extends to very high energies with the powerlaw photon index of 1.49 ^ { +0.04 } _ { -0.03 } .
Such a flat powerlaw component has never been reported from Cyg X-3 .
Interestingly the fitted powerlaw model in 25-70 keV , when extrapolated to the radio frequency , predicts the radio flux density consistent with the trend measured from RATAN-600 telescope at 11.2 GHz .
This provides a direct evidence of the synchrotron origin of flat X-ray powerlaw component and the most extensive monitoring of the broadband X-ray behavior at the moment of decoupling the giant radio jet base from the compact object in Cyg X-3 .
Using SXT and LAXPC observations , we determine the giant flare ejection time as MJD 57845.34 \pm 0.08 when 11.2 GHz radio flux density increases from \sim 100 to \sim 478 mJy .