2024MNRAS.531.1149N

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 $\nu _{_{\rm QPO}}=1.43$ Hz, a coherence factor Q = 7.14 and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95{{\ \rm per\ cent}}$ 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 ($\sim 12\!-\!3{{\ \rm per\ cent}}$) with energy. Further, $\nu _{_{\rm QPO}}$ is seen to correlate (anticorrelate) 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 per cent) in presence of a 'weak' signature of disc emissions (kT_in ∼ 0.36 keV) indicating the harder spectral distribution. Considering source mass $M_{\rm BH}=10\, \mathrm{M}_\odot$ and distance 1.5 < d (kpc) < 5, the unabsorbed bolometric luminosity is estimated as $\sim 0.03\!-\!0.92{{\ \rm per\ cent}}\, L_{\rm Edd}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.