A deep ROSAT PSPC observation towards the CMB decrement close to PC1643+4631 A & B: no cluster X-ray emission

We report on a 16-ks observation with the ROSAT PSPC centred on the cosmic microwave background (CMB) decrement which Jones et al. report in the direction of the quasar pair PC1643+4631 A&B. 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 X-ray flux (0.5-2.0 keV) within a circle of radius 1.5 arcmin is 1.0 × 10−14 erg cm−2 s−1 at 99.7 per cent significance. 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 per cent confidence for a 10-keV isothermal object of any size. Even higher limits obtain for objects with temperatures between 0.2 and 10 keV. A discussion of our result shows that an explanation other than Comptonization by hot gas within a virialized, cluster-like object, appears to be needed.