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\ 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 rms_QPO decreases (~ 12\!-\!3\ per\ cent) with energy. Further, ν _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 (kTin ∼ 0.36 keV) indicating the harder spectral distribution. Considering source mass MBH=10\, M_ and distance 1.5 < d (kpc) < 5, the unabsorbed bolometric luminosity is estimated as ~ 0.03\!-\!0.92\ per\ cent\, LEdd. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.