Time-Resolved Hubble Space Telescope UV Observations of an X-Ray Quasi-Periodic Eruption Source

X-ray quasi-periodic eruptions (QPEs) are a novel mode of variability in nearby galactic nuclei whose origin remains unknown. Their multi-wavelength properties are poorly constrained, as studies have focused almost entirely on the X-ray band. Here we report on time-resolved, coordinated _Hubble Space Telescope_ far ultraviolet and _XMM-Newton_ X-ray observations of the shortest period X-ray QPE source currently known, eRO-QPE2. We detect a bright UV point source (\(L_{FUV} \approx \text{few} \times 10^{41}\) erg s-1) that does not show statistically significant variability between the X-ray eruption and quiescent phases. This emission is unlikely to be powered by a young stellar population in a nuclear stellar cluster. The X-ray-to-UV spectral energy distribution can be described by a compact accretion disk (\(R_{out} = 343_{-138}^{+202}R_g\)). Such compact disks are incompatible with typical disks in active galactic nuclei, but form naturally following the tidal disruption of a star. Our results rule out models (for eRO-QPE2) invoking i) a classic AGN accretion disk and ii) no accretion disk at all. For orbiter models, the expected radius derived from the timing properties would naturally lead to disk-orbiter interactions for both quasi-spherical and eccentric trajectories. We infer a black hole mass of log\((M_{BH}) = 5.9 \pm 0.3\) M⊙ and Eddington ratio of 0.13\({}_{-0.07}^{}\); in combination with the compact outer radius this is inconsistent with existing disk instability models. After accounting for the quiescent disk emission, we constrain the ratio of X-ray to FUV luminosity of the eruption component to be \(L_X/L_{FUV} > 16 - 85\) (depending on the intrinsic extinction).