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We report on the detection of type-C quasi-periodic oscillations during theinitial stages of the outburst of Swift J1727.8-1613 in 2023. Using data of theINTEGRAL observatory along with the data of the SRG/ART-XC and Swift/XRTtelescopes the fast growth of the QPO frequency was traced. We present a hardX-ray lightcurve that covers the initial stages of the 2023 outburst - the fastrise and plateau - and demonstrate that the QPO frequency was stable during theplateau. The switching from type-C to type-B QPO was detected with thebeginning of the source flaring activity. We have constructed a broad-bandspectrum of Swift J1727.8-1613 and found an additional hard power-law spectralcomponent extending at least up to 400 keV. Finally, we have obtained an upperlimit on the hard X-ray flux at the beginning of the optical outburst andestimated the delay of the X-ray outburst with respect to the optical one.

Hard X-rays and QPO in Swift J1727.8-1613: the rise and plateau of the 2023 outburst