In an intriguing paper Jones et al. (1997) reported the discovery of a decrement in the cosmic microwave background (CMB) in the direction of the quasar pair PC1643+4631 A & B, which they suggest is due to a cluster at and might also be responsible for the double image of a single quasar (see also
Cotter, these proceedings). We present results from a pointed ROSAT observation which investigates this hypothesis.
The observed decrement in peak flux density is , corresponding to a minimum magnitude of a CMB temperature decrement of for a spherical King model with angular core radius . Primordial fluctuations in the CMB can not account for the amplitude of the effect. As a decrement, it can not be due to undetected sources and the Rees-Sciama effect of the non-linear gravitational collapse of an object seems too small to account for the full effect (see, for example, Dabrowski, these proceedings). In an extensive search no optical or infrared counterpart was found either (Saunders et al. (1997)).
The plausible explanation, given in the paper by Jones et al. (1997), is that the decrement is due to the Sunyaev & Zel'dovich (1972) (SZ-) effect, inverse Compton scattering of CMB photons, by hot cluster gas. A comparison done in their paper with a serendipitous 11.4 ks ROSAT PSPC observation where the position unfortunately lies near to the edge of the field of view, showed no emission by the cluster gas with a bolometric flux limit of , which constrained the redshift of the cluster to values greater than . We checked the position in the ROSAT All-Sky Survey and found independently a comparable bolometric flux limit of .
Another discovery of a decrement (Richards et al. (1997), see also Partridge, these proceedings), but with a smaller amplitude and on a smaller angular scale, also without strong X-ray emission (from a 100 ks ROSAT HRI observation) was reported recently, which could indicate that these decrements are a common class of objects.
Since X-ray emission offers the only direct observation of the gas causing a decrement in the CMB, and since the ROSAT detector in the energy range of 0.1--2.4 keV is, because of the redshift dependence with energy of the luminosity density, ideally suited to observe a high redshift cluster with intrinsic temperature 5--10 keV, we proposed a pointed ROSAT observation.