Bias from gas inhomogeneities in the pressure profiles as measured from X-ray and Sunyaev–Zeldovich observations

X-ray observations of galaxy clusters provide emission measure weighted spectra, arising from a range of density and temperature fluctuations in the intra-cluster medium (ICM). This is fitted to a single temperature plasma emission model to provide an estimate of the gas density and temperature, which are sensitive to the gas inhomogeneities. Therefore, X-ray observations yield a potentially biased estimate of the thermal gas pressure, PX. At the same time Sunyaev-Zeldovich (SZ) observations directly measure the integrated gas pressure, PSZ. If the X-ray pressure profiles are strongly biased with respect to the SZ, then one has the possibility to probe the gas inhomogeneities (their amplitude and physical nature), even at scales unresolved by the current generation of telescopes. At the same time, a weak bias has implications for the interchangeable use of mass proxies like YSZ and YX as cosmological probes. In this paper we investigate the dependence of the bias, defined as bP(r) ≡ PX(r)/PSZ(r)−1, on the characteristics of fluctuations in the ICM taking into account the correlation between temperature and density fluctuations. We made a simple prediction of the irreducible bias in idealised X-ray vs SZ observations using multi-temperature plasma emission model. We also provide a simple fitting form to estimate the bias given the distribution of fluctuations. In real observations there can be additional complications arising from instrumental background, insufficient photon statistics, asphericity, method of deprojection, etc. Analysing a sample of 16 clusters extracted from hydrodynamical simulations, we find that the median value of bias is within ±3% within R500, it decreases to −5% at R500 < r < 1.5 R500 and then rises back to ∼ 0% at r & 2 R500. The scatter of bP(r) between individual relaxed clusters is small – at the level of < 0.03 within R500, but turns significantly larger (0.25) and highly skewed (bP(r) ≫ 0) at r & 1.5 R500. For any relaxed cluster we find |bP(r)| < 15% within R500, across different implementations of input physics in the simulations. Unrelaxed clusters exhibit a larger scatter in bP(r) (both from radius to radius and from cluster to cluster).