Discovery of evolving low-frequency QPOs in hard X-rays (~ 100 keV) observed in black hole Swift J1727.8-1613 with $AstroSat$

We report the first detection of evolving Low-Frequency Quasi-periodic Oscillation (LFQPO) frequencies in hard X-rays upto $100$ keV with $AstroSat/LAXPC$ during `unusual' outburst phase of Swift J1727.8-1613 in hard-intermediate state (HIMS). The observed LFQPO in $20 - 100$ keV has a centroid ν_QPO=1.43 Hz, a coherence factor $Q= 7.14$ and an amplitude rms_QPO = 10.95% with significance σ = 5.46. Type-C QPOs ($1.09-2.6$ Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays ($80 - 100$ keV), where rms_QPO decreases (~ 12-3%) with energy. Further, ν_QPO is seen to correlate (anti-correlate) with low (high) energy flux in $2-20$ keV ($15-50$ keV). Wide-band ($0.7 - 40$ keV) energy spectrum of $NICER/XTI$ and $AstroSat/LAXPC$ is satisfactorily described by the `dominant' thermal Comptonization contribution (~ 88%) in presence of a `weak' signature of disk emissions (kTᵢₙ ~ 0.36 keV) indicating the harder spectral distribution. Considering source mass MBH=10M_ and distance 1.5 < d~(kpc) < 5, the unabsorbed bolometric luminosity is estimated as ~ 0.03-0.92%LEdd. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.