3.2. SZ data

The SZ effect of CL 0016+16 has been reported many times using a number of different techniques: the large amplitude and angular size of the effect make it a useful test case for new SZ observational programmes. Three recent results are reported in Table II. Of these measurements, one used a large single-dish system equipped with a dual-beam radiometer (Hughes and Birkinshaw 1998), and the other two used interferometers designed for other purposes which had been retro-fitted to provide high sensitivity on large angular scales (Reese 2000; Grainge 2002).

Table II. SZ effect data for CL 0016

Paper	Telescope	TRJ (mK)
Hughes and Birkinshaw (1998)	OVRO 40-m telescope	-1.20 ± 0.19
Reese (2000)	BIMA and OVRO interferometers	-1.24 ± 0.11
Grainge (2002)	Ryle telescope	-1.08 ± 0.11
Converted to XMM model		-1.26 ± 0.07

Although CL 0016+16 has one of the largest SZ effects, in terms of the central surface brightness, the measurements have relatively low signal/noise compared with the X-ray data. model fits to the SZ data are consistent with the fits obtained from the X-ray data, but add little weight towards improving the errors on the derived structural parameters. Thus it is better to use the X-ray derived structural parameters to fit the SZ data (or to undertake a simultaneous SZ and X-ray fit) than to perform independent fits.

Since the three SZ effects reported in Table II were extracted from the raw data via the application of a different model for the gas, and each model is somewhat different from the model fitted from the X-ray data, it is necessary to correct them to the same overall measure of the SZ effect. The overall result after scalings that take account of the different efficiencies of the observing techniques is that the central SZ effect, at zero frequency, is -1.26 ± 0.07 mK.