The Enhancement and Decrement of the Sunyaev-Zeldovich Effect toward the ROSAT Cluster RX J0658–5557*

We report simultaneous observations of the X-ray cluster RX J0658-5557 at 1.2 and 2 mm made with a double-channel photometer on the Swedish ESO Submillimeter Telescope (SEST) in search of Sunyaev-Zeldovich (SZ) effect. The SZ data were analyzed using the relativistically correct expression for the Comptonization, and we find (2.60?0.79)?10?4 from the detected decrement, which is consistent with that computed using the X-ray (ROSAT and ASCA) observations. The uncertainty includes contributions from statistical uncertainties in the detection, systematics, and calibration. The 1.2 mm channel data alone give rise to a larger Comptonization parameter; this result is discussed in terms of contamination from foreground sources and/or dust in the cluster or from a possible systematic effect. We then make use of a combined analysis of the ROSAT and ASCA X-ray satellite observations to determine an isothermal model for the SZ surface brightness. Since the cluster is asymmetrical and probably in a merging process, models are only approximate. The associated uncertainty can, however, be estimated by exploring a set of alternative models. We then find a factor of 1.3 for the global uncertainty on the Comptonization parameter. Combining the SZ and the X-ray measurements, we determine a value for the Hubble constant. The 2 mm data are consistent with H0(q0=?)=53+38?28 km s-1 Mpc-1, where the uncertainty is dominated by the uncertainty in the models of the X-ray plasma halo.