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 from a typical cluster temperature profile) to $kT\sim$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\sim$6-7 keV within 10' or $\sim$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 ${\cal M}\sim$1.3. We also successfully determined the abundance profile up to 15' (1.3 Mpc), showing an almost constant value ($Z$=0.3 solar) at the cluster outskirt. We estimated the Compton $y$-parameter to be $\sim$14.5$\pm1.3\times10^{-6}$, which is in agreement with $Planck$'s results (14-17$\times10^{-6}$ on the filament). The line of sight depth of the filament is $l\sim$1.1 Mpc, indicating that the geometry of filament is likely a pancake shape rather than cylindrical. The total mass of the filamentary structure is $\sim$7.7$\times10^{13}~\rm M_{\odot}$. 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.
Comment: 8 pages, 8 figures, accepted for publication in A&A