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We study a merger of the NGC 4839 group with the Coma cluster using X-rayobservations from the XMM-Newton and Chandra telescopes. X-ray data show twoprominent features: (i) a long (~600 kpc in projection) and bent tail of coolgas trailing (towards south-west) the optical center of NGC 4839, and ii) a"sheath" region of enhanced X-ray surface brightness enveloping the group,which is due to hotter gas. While at first glance the X-ray images suggest thatwe are witnessing the first infall of NGC 4839 into the Coma cluster core, weargue that a post-merger scenario provides a better explanation of the observedfeatures and illustrate this with a series of numerical simulations. In thisscenario, the tail is formed when the group, initially moving to thesouth-west, reverses its radial velocity after crossing the apocenter, the rampressure ceases and the ram-pressure-displaced gas falls back toward the centerof the group and overshoots it. Shortly after the apocenter passage, theoptical galaxy, dark matter and gaseous core move in a north-east direction,while the displaced gas continues moving to the south-west. The "sheath" isexplained as being due to interaction of the re-infalling group with its owntail of stripped gas mixed with the Coma gas. In this scenario, the shock,driven by the group before reaching the apocenter, has already detached fromthe group and would be located close to the famous relic to the south-west ofthe Coma cluster.

Close-up view of an ongoing merger between the NGC 4839 group and the Coma cluster - a post-merger scenario