2025A&A...693A..55L

Massive clusters of galaxies are very rare in the observable Universe. Mergers of such clusters observed close to pericenter passage are even rarer. Here, we report on one such case: The massive (∼10¹⁵ M_⊙) and hot (kT ∼ 10 keV) cluster CL0238.3+2005 at z = 0.42. For this cluster, we combined X-ray data from SRG/eROSITA and Chandra, optical images from DESI, and spectroscopy from the BTA and RTT-150 telescopes. The X-ray and optical morphologies suggest an ongoing merger with a projected separation of the subhalos of ∼200 kpc. The line-of-sight velocity of galaxies that are tentatively associated with the two merging halos differs by 2000-3000 km s^–1. We conclude that the merger axis is most likely neither close to the line of sight nor to the sky plane. We compare CL0238 with the two well-known clusters MACS0416 and the Bullet and conclude that CL0238 corresponds to an intermediate phase between the pre-merging MACS0416 cluster and the post-merger Bullet cluster. Namely, this cluster recently (only ≲0.1 Gyr ago) experienced an almost head-on merger. We argue that this “just after” system is a very rare case and an excellent target for lensing, the Sunyaev-Zeldovich effect, and X-ray studies that can constrain properties ranging from dynamics of mergers to self-interacting dark matter, and plasma effects in the intracluster medium that are associated with shock waves, for instance, electron-ion equilibration efficiency and relativistic particle acceleration.