Energy-dependent Optical/Near-infrared and X-ray Correlations in Swift J1727.8–1613

Abstract We present a timing analysis of the black hole transient Swift J1727.8–1613 during its intermediate state. We use coordinated broadband X-ray observations from Insight-HXMT (2–150 keV), together with optical data from ULTRACAM (gs and is bands) and near-infrared data from HAWK-I (Ks band), obtained on 2023 September 9. As shown by previous studies, the Fourier power spectrum shows a strong quasi-periodic oscillation (QPO) in the Ks, is and X-ray bands. Cross-correlation analysis reveals a complex coupling between the optical/near-infrared (OIR) and X-ray emission, including a delayed optical anti-correlation, a strong infrared correlation, and a pronounced dependence of these features on X-ray energy, suggesting multiple Comptonisation regions. In contrast, the lag properties do not change at the QPO frequency, displaying an OIR lag of ∼60–80 ms up to 150 keV. We discuss these results in the context of small-scale jet and hot accretion flow scenarios.