Discovery of evolving low-frequency QPOs in hard X-rays ($\sim 100$ 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 ${\nu }_{{}_{\mathrm{QPO}}}=1.43$ Hz, a coherence factor $Q=7.14$ and an amplitude $\mathrm{rm}{\mathrm{s}}_{{}_{\mathrm{QPO}}}=10.95\%$ with significance $\sigma =5.46$. 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 $\mathrm{rm}{\mathrm{s}}_{{}_{\mathrm{QPO}}}$ decreases ($\sim 12-3\%$) with energy. Further, ${\nu }_{{}_{\mathrm{QPO}}}$ 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 ($\sim 88\%$) in presence of a `weak' signature of disk emissions ($k{T}_{\mathrm{in}}\sim 0.36$ keV) indicating the harder spectral distribution. Considering source mass $M_{\mathrm{BH}}=10M_{\odot }$ and distance $1.5<\mathrm{d}(\mathrm{kpc})<5$, the unabsorbed bolometric luminosity is estimated as $\sim 0.03-0.92\%L_{\mathrm{Edd}}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.

10 pages, 5 figures, 3 tables, accepted for publication in MNRAS