Discovery of evolving low-frequency QPOs in hard X-rays ($\sim 100$ 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 $AstroSat\mathrm{/}LAXPC$ during unusual' outburst phase of Swift J1727.8-1613 in hard-intermediate state (HIMS). The observed LFQPO in $20 - 100$ keV has a centroid $ν_{_{\rm QPO}}=1.43$ Hz, a coherence factor $Q= 7.14$ and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95\%$ 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 ${\rm rms_{_{\rm QPO}}}$ decreases ($\sim 12-3\%$) with energy. Further, $ν_{_{\rm QPO}}$ 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 ($\sim 88\mathrm{\%}$) in presence of a `weak’ signature of disk emissions ($k{T}_{\mathrm{i}\mathrm{n}}\sim 0.36$ keV) indicating the harder spectral distribution. Considering source mass $M_{\mathrm{B}\mathrm{H}}=10M_{\odot }$ and distance $1.5<\mathrm{d}(\mathrm{k}\mathrm{p}\mathrm{c})<5$, the unabsorbed bolometric luminosity is estimated as $\sim 0.03-0.92\mathrm{\%}{L}_{\mathrm{E}\mathrm{d}\mathrm{d}}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.