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=½)=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.