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

TL;DR

This paper reports the detection and analysis of type-C QPOs during the 2023 outburst of Swift J1727.8-1613, revealing spectral and timing evolution, including a transition to type-B QPOs and a hard X-ray component up to 400 keV.

Contribution

It provides the first detailed timing and spectral analysis of the 2023 outburst, including QPO evolution and broad-band spectral components of Swift J1727.8-1613.

Findings

QPO frequency grew rapidly during the initial outburst stages

A stable QPO frequency was observed during the plateau phase

An additional hard power-law component extended up to 400 keV

Abstract

We report on the detection of type-C quasi-periodic oscillations during the initial stages of the outburst of Swift J1727.8-1613 in 2023. Using data of the INTEGRAL observatory along with the data of the SRG/ART-XC and Swift/XRT telescopes the fast growth of the QPO frequency was traced. We present a hard X-ray lightcurve that covers the initial stages of the 2023 outburst - the fast rise and plateau - and demonstrate that the QPO frequency was stable during the plateau. The switching from type-C to type-B QPO was detected with the beginning of the source flaring activity. We have constructed a broad-band spectrum of Swift J1727.8-1613 and found an additional hard power-law spectral component extending at least up to 400 keV. Finally, we have obtained an upper limit on the hard X-ray flux at the beginning of the optical outburst and estimated the delay of the X-ray outburst with respect to the optical one.