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

Abstract 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 $\unicode{x03C1} \rightarrow \unicode{x03BA}$ class transition of the source. This new ‘unknown’ class ( $\tau$ ) is characterised 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 colour-colour 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 ( $kT_\textrm{e}\sim 1.7$ 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 ( $L_\textrm{bol}$ ) to be about $42\% L_\textrm{Edd}$ within 1–100 keV, indicating the sub-Eddington accretion regime of the source.