Energy-dependent optical/near-infrared and X-ray correlations in Swift J1727.8–1613
Ruican Ma, Federico Vincentelli, Diego Altamirano, Alexandra Veledina, Poshak Gandhi, Piergiorgio Casella, Tariq Shahbaz, Sian Woahene-DemehinABSTRACT
We present a timing analysis of the black hole transient Swift J1727.8–1613 during its intermediate state. We use coordinated broad-band X-ray observations from Insight–HXMT (2–150 keV), together with optical data from ULTRACAM ($g_s$ and $i_s$ bands) and near-infrared data from HAWK-I ($K_s$ band), obtained on 2023 September 9. As shown by previous studies, the Fourier power spectrum shows a strong quasi-periodic oscillation (QPO) in the $K_s$, $i_s$, 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 anticorrelation, a strong infrared correlation, and a pronounced dependence of these features on X-ray energy, suggesting multiple Comptonization regions. In contrast, the lag properties do not change at the QPO frequency, displaying an OIR lag of $\sim$60–80 ms up to 150 keV. We discuss these results in the context of small-scale jet and hot accretion flow scenarios.