2017A&A...606A...1A

We report on the results of a Suzaku observation of the plasma in the filament located between the two massive clusters of galaxies, Abell 399 and Abell 401. Abell 399 (z=0.0724) and Abell 401 (z=0.0737) are expected to be in the initial phase of a cluster merger. In the region between the two clusters, we find a clear enhancement in the temperature of the filament plasma from 4keV (expected value of a typical cluster temperature profile) to kT∼6.5keV. Our analysis also shows that filament plasma is present out to a radial distance of 15' (1.3Mpc) from a line connecting the two clusters. The temperature profile is characterized by an almost flat radial shape with kT∼6-7keV within 10' or ∼0.8Mpc. Across r=8' from the axis, the temperature of the filament plasma shows a drop from 6.3keV to 5.1keV, indicating the presence of a shock front. The Mach number based on the temperature drop is estimated to be M∼1.3. We also successfully determined the abundance profile up to 15' (1.3Mpc), showing an almost constant value (Z=0.3 solar) at the cluster outskirts. We estimated the Compton y parameter to be ∼14.5±1.3x10^–6, which is in agreement with the Planck results (14-17x10^–6 on the filament). The line-of-sight depth of the filament is l∼1.1Mpc, which indicates that the geometry of filament is likely a pancake shape rather than cylindrical. The total mass of the filamentary structure is ∼7.7x10¹³M_☉. We discuss a possible interpretation of the drop of X-ray emission at the rim of the filament, which was pushed out by the merging activity and formed by the accretion flow induced by the gravitational force of the filament.