Discovery of a Premerger Shock in an Intercluster Filament in Abell 98

We report the first unambiguous detection of an axial merger shock in the early-stage merging cluster Abell 98 using deep (227 ks) Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of ${ \mathcal M }$ ≈ 2.3 ± 0.3. Our discovery of the axial merger shock front unveils a critical epoch in the formation of a massive galaxy cluster, when two subclusters are caught in the early phase of the merging process. We find that the electron temperature in the postshock region favors the instant collisionless model, where electrons are strongly heated at the shock front, by interactions with the magnetic field. We also report on the detection of an intercluster gas filament, with a temperature of kT = 1.07 ± 0.29 keV, along the merger axis of Abell 98. The measured properties of the gas in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm–hot intergalactic medium (WHIM), where WHIM filaments interface with the virialization regions of galaxy clusters.