The Sunyaev-Zel'dovich (SZ) effect spectrum crosses through a null where ΔT CMB = 0 near ν0 = 217?GHz. In a cluster of galaxies, ν0 can be shifted from the canonical thermal SZ effect value by corrections to the SZ effect scattering due to the properties of the inter-cluster medium. We have measured the SZ effect in the hot galaxy cluster RX J 1347.5 – 1145 with Z-Spec, an R ~ 300 grating spectrometer sensitive between 185 and 305?GHz. These data comprise a high spectral resolution measurement around the null of the SZ effect and clearly exhibit the transition from negative to positive ΔT CMB over the Z-Spec band. The SZ null position is measured to be ν0 = 225.8 ± 2.5(stat.) ± 1.2(sys.)?GHz, which differs from the canonical null frequency by 3.0σ and is evidence for modifications to the canonical thermal SZ effect shape. Assuming the measured shift in ν0 is due only to relativistic corrections to the SZ spectrum, we place the limit kT e = 17.1 ± 5.3?keV from the zero-point measurement alone. By simulating the response of the instrument to the sky, we are able to generate likelihood functions in {y 0, T e, v pec} space. For v pec = 0?km?s–1, we measure the best-fitting SZ model to be y 0 = 4.6+0.6 – 0.9 × 10–4, T e, 0 = 15.2+12 – 7.4?keV. When v pec is allowed to vary, a most probable value of v pec = + 450 ± 810?km?s–1 is found.
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