a contribution from particles hitting the XIS detectors that are not focused by the telescope and three instrumental lines. For the particle spectrum, we include a power law where both the normalization and slope can vary. The instrumental lines include an Al K line at 1.49 keV, an Si K line at 1.74 keV, and an Au M line at 2.12 keV.[^5] We model each of these lines as Gaussians with widths and normalizations left to vary as free parameters and the centroids fixed. This model component is only folded through the RMF response (as opposed to both the RMF and ARF), and it contributes to both the observed and NXB spectra.

The CXB spectrum is typically modeled as an absorbed power law or multiple broken power laws, and is thought to be due to unresolved active galactic nuclei (AGNs). The differences between these spectral shapes have minimal effects on the measured oxygen line intensities, since the CXB contributes $\lesssim10\%$ to the total SXRB flux below $\approx$1 keV. Therefore, we adopt the spectral shape used by , which is a power law with fixed spectral index of 1.46 [@chen_etal97]. These authors discuss the uncertainty in the CXB power law normalization, 