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

ABSTRACT 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 _ ₐ₏₎=1. 43 Hz, a coherence factor Q = 7. 14 and an amplitude rms_{ ₐ₏₎} = 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_{ ₐ₏₎} decreases (12\!-\!3{\ per\ cent}) with energy. Further, _ ₐ₏₎ 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 M ₁₇=10\, M_ and distance 1. 5 d (kpc) 5, the unabsorbed bolometric luminosity is estimated as 0. 03\!-\!0. 92{\ per\ cent}\, L ₄₃₃. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.