Possible detection of HFQPOs associated with 'unknown' variability class of GRS 1915+105

We present a comprehensive spectro-temporal analysis of GRS $1915+105$ observed with AstroSat during June, $2017$. A detailed study of the temporal properties reveals the appearance of an `unknown' variability class ($\tau$) during $\rho \rightarrow \kappa$ class transition of the source. This new `unknown' class ($\tau$) is characterized by the irregular repetition of low count `dips' along with the adjacent `flare' like features in between two successive steady count rate durations, resulting in uniform `$C$' shaped distribution in the color-color diagram. A detailed comparative study of the variability properties between the $\tau$ class and other known variability classes of GRS $1915+105$ indicates it as a distinct variability class of the source. Further, we find evidence of the presence of possible HFQPO features at $\sim 71$ Hz with quality factor $\sim 13$, rms amplitude $\sim 4.69\%$, and significance $3\sigma$, respectively. In addition, a harmonic-like feature at $\sim 152$ Hz is also seen with quality factor $\sim 21$, rms amplitude $\sim 5.75\%$ and significance $\sim 4.7\sigma$. The energy-dependent power spectral study reveals that the fundamental HFQPO and its harmonic are present in $3-15$ keV and $3-6$ keV energy ranges, respectively. Moreover, the wide-band ($0.7-50$ keV) spectral modelling comprising of thermal Comptonization component indicates the presence of a cool ($kT_{\rm e}\sim 1.7$ keV) and optically thick (optical depth $\sim 14$) Comptonizing `corona', which seems to be responsible in regulating the HFQPO features in GRS $1915$+$105$. Finally, we find the bolometric luminosity ($L_{\rm bol}$) to be about $42\% L_{\rm Edd}$ within $1-100$ keV, indicating the sub-Eddington accretion regime of the source.