2025ApJ...985..146G

We present an analysis of Hubble Space Telescope (HST) and XMM-Newton data of the tidal disruption event (TDE) candidate and quasi-periodic eruption (QPE) source GSN 069. Using ultraviolet (UV) and optical images at HST resolution, we show that GSN 069's emission consists of a point source superimposed on a diffuse stellar component. The latter accounts for ≤5% of the UV emission in the inner 0$\mathop{.}\limits^{\unicode{x02033}}$5 × 0$\mathop{.}\limits^{\unicode{x02033}}$5 region, while the luminosity of the former cannot be attributed to stars. Analyzing the 2014/2018 HST UV spectra, we show that to leading order the intrinsic spectral shape is νL_ν ∝ ν^4/3, with ∼10% far-UV flux variability between epochs. The contemporaneous X-ray and UV spectra can be modeled self-consistently in a thin disk framework. At observed epochs, the disk had an outer radius (R_out) of ${ \mathcal O }(1{0}^{3}{R}_{{\rm{g}}})$, showing both cooling and expansion over 4 yr. Incorporating relativistic effects via numerical ray tracing, we constrain the disk inclination angle (i) to be 30^∘ ≲ i ≲ 65^∘ and identify a narrow region of spin-inclination parameter space that describes the observations. These findings confirm that GSN 069 hosts a compact, viscously expanding accretion disk likely formed after a TDE. The implications for QPE models are (i) that no published disk-instability model can explain the disk's stability in 2014 (no QPEs) and its instability in 2018 (QPEs present), and (ii) while the disk size in 2018 allows for orbiter-disk interactions to produce QPEs, in 2014 the disk was already sufficiently extended, yet no QPEs were present. These findings pose challenges to existing QPE models.