Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion

Abstract: The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the Solar and Heliosphere Observatory (SOHO) observed the hydrogen coma of interstellar comet 3I/ATLAS, also called C/2025 N1 (ATLAS), beginning on November 6, 2025, 9 days after perihelion. Water production rates were calculated from each image of 3I/ATLAS using the methodology of Mäkinen & Combi (2005, Icarus 177, 217) and fluorescence rates calculated using the daily solar Lyman-alpha fluxes from the LASP database corrected for solar rotation. The method has been used for over 90 comet apparitions (Combi 2022; Combi et al 2019). A water production rate of 3.17 x 10^29 s^-1 was found on November 6 when the comet was at a heliocentric distance of 1.40 au and at a sufficient solar elongation angle. It decreased over time after that, down to 1-2 x 10^28 s^-1 around 40 days post-perihelion (December 8).