We report a significant detection of the hot intergalactic medium in the filamentary bridge connecting the galaxy clusters Abell 399 and Abell 401. This result is enabled by a low-noise, high-resolution map of the thermal Sunyaev-Zeldovich signal from the Atacama Cosmology Telescope (ACT) and Planck satellite. The ACT data provide the $1.65$ resolution that allows us to clearly separate the profiles of the clusters, whose centres are separated by $37$, from the gas associated with the filament. A model that fits for only the two clusters is ruled out compared to one that includes a bridge component at $>5sigma$. Using a gas temperature determined from Suzaku X-ray data, we infer a total mass of $(3.3pm0.7)times10^{14},mathrm{M}_{odot}$ associated with the filament, comprising about $8%$ of the entire Abell 399-Abell 401 system. We fit two phenomenological models to the filamentary structure; the favoured model has a width transverse to the axis joining the clusters of ${sim}1.9,mathrm{Mpc}$. When combined with the Suzaku data, we find a gas density of $(0.88pm0.24)times10^{-4},mathrm{cm}^{-3}$, considerably lower than previously reported. We show that this can be fully explained by a geometry in which the axis joining Abell 399 and Abell 401 has a large component along the line of sight, such that the distance between the clusters is significantly greater than the $3.2,mathrm{Mpc}$ projected separation on the plane of the sky. Finally, we present initial results from higher resolution ($12.7$ effective) imaging of the bridge with the MUSTANG-2 receiver on the Green Bank Telescope.
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