We have obtained a 12.5 ksec image of the Hercules Cluster , A2151 , with the ROSAT PSPC .
Comparison of the optical and X-ray data suggest the presence of at least three distinct subclusters in A2151 .
The brightest X-ray emission coincides with the highest-density peak in the galaxy distribution , and is bimodal .
The northern subclump , distinct in position and velocity , has no detectable X-ray gas .
The eastern subclump , apparent in the optical contour map , is indistinguishable from the main clump in velocity space but is clearly visible in the X-ray image .

X-ray spectra derived from the central peak of emission yield a best-fit temperature of 1.6 keV .
The emission coincident with the eastern clump of galaxies is cooler , 0.8 keV , and is outside the 90 % confidence intervals of the central peak temperature .

We suggest that the eastern and central subclusters have recently undergone a merger event .
The lack of X-ray emission to the north suggests that those galaxies do not form a physically-distinct structure ( i.e .
they are not located within a distinct gravitational potential ) , but rather that they are falling into the cluster core along the filament defined by the Hercules Supercluster .