A DEEP ROSAT PSPC OBSERVATION TOWARDS THE CMB DECREMENT CLOSE TO PC1643-A AND PC1643+4631-B - NO CLUSTER X-RAY-EMISSION(4631)

We report on a 16-ks observation with the ROSAT PSPC centred on the co smic microwave background (CMB) decrement which Jones et al. report in the direction of the quasar pair PC1643+4631 A&B. We do not detect th e 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 X-ray flux (0.5-2.0 keV) within a circle of radius 1.5 arcmin is 1.0 x 10(-14) erg cm(-2) s(-1) at 99.7 per cent significance. We inves tigate 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 per cent confidence for a 10-keV isothermal object of any size. Even higher li mits obtain for objects with temperatures between 0.2 and 10 keV. A di scussion of our result shows that an explanation other than Comptoniza tion by hot gas within a virialized, cluster-like object, appears to b e needed.