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We report the results of AstroSat observations of GRS 1915$+$105 obtainedusing 100 ks guaranteed-time (GT) during the soft state. The Color-ColorDiagram (CCD) indicates a variability class of $\delta$ with the detection ofHigh Frequency QPO (HFQPO) in the power density spectra (PDS). The HFQPO isseen to vary in the frequency range of $67.96 - 70.62$ Hz with percentage rms$\sim 0.83 - 1.90$ % and significance varying from $1.63 - 7.75$. The energydependent power spectra show that the HFQPO features are dominant only in $6 -25$ keV energy band. The broadband energy spectra ($0.7 - 50$ keV) of SXT (SoftX-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) modelledwith nthComp and powerlaw imply that the source has an extended corona inaddition to a compact 'Comptonizing corona' that produces high energy emissionand exhibits HFQPOs. The broadband spectral modelling indicates that the sourcespectra are well described by thermal Comptonization with electron temperature(kT$_{\rm e}$) of $2.07 - 2.43$ keV and photon-index ($\Gamma_{\rm nth}$)between $1.73-2.45$ with an additional powerlaw component of photon-index($\Gamma_{\rm PL}$) between $2.94 - 3.28$. The norm of nthComp component ishigh ($\sim 8$) during the presence of strong HFQPO and low ($\sim 3$) duringthe absence of HFQPO. Further, we model the energy spectra with the kerrbbmodel to estimate the accretion rate, mass and spin of the source. Our findingsindicate that the source accretes at super-Eddington rate of $1.17-1.31~\dot{M}_{\rm Edd}$. Moreover, we find the mass and spin of the source as $12.44- 13.09~M_{\odot}$ and $0.990-0.997$ with $90\%$ confidence suggesting that GRS1915$+$105 is a maximally rotating stellar mass X-ray binary black hole source.

AstroSat view of GRS 1915+105 during the Soft State: Detection of HFQPOs and estimation of Mass and Spin