Exotic optical variability in the black hole X-ray binary IGR J17091 − 3624

ABSTRACT IGR J17091 − 3624 is a distinctive black hole X-ray binary exhibiting exotic variability, including complex ‘heartbeat’ oscillations in its X-ray light curves, similar to those observed in GRS 1915 + 105, a system renowned for its structured, rapid X-ray variability but heavily obscured at optical wavelengths. In contrast, IGR J17091 − 3624 is less obscured, making it a more accessible target for optical investigations. Due to its weak radio emission, optical and infrared data are essential to probe the jet and outer disc behaviour of IGR J17091 − 3624. This study presents the first long-term optical monitoring of IGR J17091 − 3624, using data from the Las Cumbres Observatory over its 2011, 2016, and 2022 outbursts. We combine these observations with quasi-simultaneous X-ray data from Swift/XRT, RXTE, and NICER, employing light curve and variability analysis, spectral energy distributions, colour–magnitude diagrams, and optical/X-ray correlations to investigate optical emission mechanisms. We find that the optical and X-ray fluxes are significantly correlated, following a power-law relation ($F_{\mbox{opt}} \propto F_{\rm X}^{0.40\pm 0.04}$), suggesting that the optical emission in IGR J17091 − 3624 is dominated by an X-ray-irradiated accretion disc. Based on optical spectral slope constraints, we estimate the extinction towards IGR J17091 − 3624 as $A_{V} =$ 4.3–6.6 mag, which translates to $N_{\mbox{H}}$ = 1.3–1.9 $\times 10^{22}\, \rm cm^{-2}$. The global optical/X-ray correlation suggests a distance estimate of 8–17 kpc, in line with previous findings. High-cadence optical observations show tentative evidence of optical oscillations that may arise from reprocessed X-ray modulations, although confirming this will require higher time-resolution optical data.