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

TL;DR

This study presents extensive broadband X-ray observations of Cyg X-3 during the formation of a giant radio jet, revealing a novel flat powerlaw component linked to synchrotron emission and providing insights into jet ejection timing.

Contribution

It reports the first detection of a flat X-ray powerlaw component in Cyg X-3 associated with jet formation, linking X-ray spectral changes to radio flare onset.

Findings

Detection of a flat powerlaw component extending to high energies.

Correlation between X-ray spectral features and radio flux increase.

Precise timing of the giant radio jet ejection.

Abstract

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. 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 +/- 0.08 when 11.2 GHz radio flux density increases from ~100 to ~478 mJy.