We report on a 16 ks observation with the ROSAT PSPC centred on the CMB decrement which Jones et al .
( 1997 ) report in the direction of the quasar pair PC1643+4631 .
We do not detect the X-ray emission which would be expected if the decrement were caused by Compton up-scattering of CMB photons by hot gas .
Our upper limit on the bolometric X-ray flux within a circle of radius 1 \aas@@fstack { \prime } 5 is 1.9 \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } at 99.7 % significance .
This is based on data in the energy range 0.5-2.0 keV and assumes bremsstrahlung with an observed temperature of 2.5 keV .
We investigate the requirements for a spherical body of hot gas to produce the decrement while remaining consistent with our data .
With conservative assumptions we establish a lower redshift limit of 2.8 at 95 % confidence for a 10 keV isothermal object of any size .
Even higher limits obtain for objects with temperatures between 0.2 and 10 keV .
We conclude that the CMB decrement is unlikely to be caused by a galaxy cluster .
We discuss alternatives to a single , spherical cluster , such as a double or elongated cluster , a supercluster/filament , or scattering by a moving cluster .
None of these appears attractive , especially since another candidate for a decrement near a QSO pair and without an obvious associated cluster has been found .
An explanation other than Comptonization by hot gas within a virialised object appears needed .