  Two long AstroSat Soft X-ray Telescope observations were taken of the third
recorded outburst of the Symbiotic Recurrent Nova, V3890 Sgr. The first
observing run, 8.1-9.9 days after the outburst, initially showed a stable
intensity level with a hard X-ray spectrum that we attribute to shocks between
the nova ejecta and the pre-existing stellar companion. On day 8.57, the first,
weak, signs appeared of Super Soft Source (SSS) emission powered by residual
burning on the surface of the White Dwarf. The SSS emission was observed to be
highly variable on time scales of hours. After day 8.9, the SSS component was
more stable and brighter. In the second observing run, on days 15.9-19.6 after
the outburst, the SSS component was even brighter but still highly variable.
The SSS emission was observed to fade significantly during days 16.8-17.8
followed by re-brightening. Meanwhile the shock component was stable leading to
increase in hardness ratio during the period of fading. AstroSat and XMM-Newton
observations have been used to study the spectral properties of V3890 Sgr to
draw quantitative conclusions even if their drawback is model-dependence. We
used the xspec to fit spectral models of plasma emission, and the best fits are
consistent with the elemental abundances being lower during the second
observing run compared to the first for spectra >1 keV. The SSS emission is
well fit by non-local thermal equilibrium model atmosphere used for white
dwarfs. The resulting spectral parameters, however, are subject to systematic
uncertainties such as completeness of atomic data.
