Water Production Rates of the Interstellar Object 3I/ATLAS

We report the detection of water activity in the third confirmed interstellar object, 3I/ATLAS, based on ultraviolet imaging with the Neil Gehrels Swift Observatory's Ultraviolet/Optical Telescope (UVOT). Assuming a reddening of 29% between 3325.7~Å and 5437.8~Å, measurements on 2025 July 31 -- August 1 yielded a first, marginal detection of OH (A²Σ -- X²Π) emission near 3085~Å, corresponding to a water production rate of (0.74 ± 0.50) × 10²⁷ molecules\,s⁻¹. The subsequent visit on 2025 August 18 -- 20 revealed a clear OH detection, implying a higher water production rate of (1.36 ± 0.35) × 10²⁷ molecules s⁻¹ (40 kg~s⁻¹) at a heliocentric distance of 2.90~au. This places 3I/ATLAS among the few comets with confirmed OH emission beyond 2.5~au, where water ice sublimation from the nucleus is typically inefficient. The inferred production rate at 2.9 au implies an active area of at least 7.8~km², assuming equilibrium sublimation. This requires that over 8% of the surface is active, which is larger than activity levels observed in most solar system comets. Contemporaneous near-infrared spectroscopy indicated the presence of icy grains in the coma, which may serve as an extended source of water vapor.