  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 1.5 arcmin
is 1.9 x 10^-14 erg 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.
