Structure formation in the current Universe operates through the accretion of group-scale systems onto massive clusters .
The detection and study of such accreting systems is crucial to understand the build-up of the most massive virialized structures we see today .
We report the discovery with XMM-Newton of an irregular X-ray substructure in the outskirts of the massive galaxy cluster Abell 2142 .
The tip of the X-ray emission coincides with a concentration of galaxies .
The bulk of the X-ray emission of this substructure appears to be lagging behind the galaxies and extends over a projected scale of at least 800 kpc .
The temperature of the gas in this region is 1.4 keV , which is a factor of \sim 4 lower than the surrounding medium and is typical of the virialized plasma of a galaxy group with a mass of a few 10 ^ { 13 } M _ { \odot } .
For this reason , we interpret this structure as a galaxy group in the process of being accreted onto the main dark-matter halo .
The X-ray structure trailing behind the group is due to gas stripped from its original dark-matter halo as it moves through the intracluster medium ( ICM ) .
This is the longest X-ray trail reported to date .
For an infall velocity of \sim 1,200 km s ^ { -1 } we estimate that the stripped gas has been surviving in the presence of the hot ICM for at least 600 Myr , which exceeds the Spitzer conduction timescale in the medium by a factor of \gtrsim 400 .
Such a strong suppression of conductivity is likely related to a tangled magnetic field with small coherence length and to plasma microinstabilities .
The long survival time of the low-entropy intragroup medium suggests that the infalling material can eventually settle within the core of the main cluster .