Properties of the cosmological filament between two clusters: A possible detection of a large-scale accretion shock by Suzaku

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 4 keV (expected value of a typical cluster temperature profile) to kT 6.5 keV. Our analysis also shows that filament plasma is present out to a radial distance of 15' (1.3 Mpc) from a line connecting the two clusters. The temperature profile is characterized by an almost flat radial shape with kT 6-7 keV within 10' or 0.8 Mpc. Across r = 8' from the axis, the temperature of the filament plasma shows a drop from 6.3 keV to 5.1 keV, 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' (1.3 Mpc), showing an almost constant value (Z = 0.3 solar) at the cluster outskirts. We estimated the Compton y parameter to be 14.5 ± 1.3 × 10^-6, which is in agreement with the Planck results (14-17 × 10^-6 on the filament). The line-of-sight depth of the filament is l 1.1 Mpc, which indicates that the geometry of filament is likely a pancake shape rather than cylindrical. The total mass of the filamentary structure is 7.7 × 10¹³M_⊙. 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.