but argue for a nominal value of 7.9 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$ at 1 keV after accounting for CXB sources with $F_X < 5 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ in the 0.5–2.0 keV band [@moretti_etal03]. We allow the CXB normalization to be a free parameter in our spectral model, but place $\pm$30% hard boundaries on the parameter to allow for field-to-field variation.

Although the oxygen lines are the measurement of interest, we still account for hot gas continuum emission in our model. We model this component as an absorbed thermal APEC plasma [@smith_etal01] with fixed solar metallicty and without the oxygen lines. We achieve the latter by setting the oxygen line emissivities to zero in the standard Xspec APEC files [@lei_etal09; @hs12]. The normalization and temperature were left as free parameters in the model. We expected the fitted plasma temperatures to be between 0.1 and 0.3 keV, but these temperatures are typically most sensitive to the oxygen lines that we disabled. Therefore, we let the plasma temperature vary outside this range, but with hard boundaries between 0.05 and 5 keV.

Our final spectral model components are the and emission lines. We model each of these components as Gaussian features 