Does the Fe L-shell blend bias abundance measurements in intermediate-temperature clusters?

Abstract

In intermediate-mass galaxy clusters (M = 2 − 4 × 1014 M⊙, or equivalently T = 2.5 − 4.5 keV), abundance measurements are almost equally driven by iron K and L transitions at ∼6.7 keV and 0.9 − 1.3 keV, respectively. While K-shell-derived measurements are considered reliable, the resolution of the currently available instrumentation, as well as our current knowledge of the atomic processes, makes the modelling of the L-line complex challenging, resulting in potential biases for abundance measurements. In this work we study with unprecedented accuracy the systematics related to the modelling of the Fe L-line complex that may influence iron-abundance measurements in the intermediate-mass range. To this end, we selected a sample of three bright and nearby galaxy clusters, with long XMM-Newton observations available and temperatures in the 2.5 − 4.5 keV range. We fit the spectra extracted from concentric rings with APEC and APEC+APEC models, by alternately excluding one band (L-shell or Kα) at a time, and derived the fractional difference of the metal abundances ΔZ/Z as an indication of the consistency between K- and L-shell-derived measurements. The ΔZ/Z distribution was then studied as a function of the cluster radius, ring temperature, and X-ray flux. The L-blend-induced systematics, measured through an individual fit of each XMM-Newton MOS and pn camera spectrum, remain constant at a 5 − 6% value in the whole 2.5 − 4.5 keV temperature range. Conversely, a joint fit of MOS and pn spectra leads to a slight excess of 1 − 2% in this estimate. No significant dependence on the ring X-ray flux is highlighted. The measured 5 − 8% value indicates a modest contribution of the systematics to the derived iron abundances, giving confidence for future measurements. To date, these findings represent the best achievable estimate of the systematics in analysis, while future microcalorimeters will significantly improve our understanding of the atomic processes underlying the Fe L emissions.