tokens: of the log N –log S distribution in this region were found to be in very good agreement with that predicted for CV populations (Fornasini et al. 2014 ). The NuSTAR survey of this region measured a log N –log S distribution that is consistent with Chandra sources if the average source spectrum is thermal with kT ≈ 15 keV (Fornasini et al. 2017 ). This is significantly harder than the kT ≈ 8 keV spectrum derived by previous soft X-ray measurements of the Galactic center (Muno et al. 2004 ; Heard Warwick 2013 ) but softer than NuSTAR measurements of the Galactic center, thus possibly indicating a lower fraction of IPs relative to other types of CVs in this region. 3. <italic>NuSTAR</italic> Data Preparation 3.1. Introduction to Zero- and Two-bounce Photons To obtain the wide solid-angle aperture necessary to study the diffuse X-ray emission from the Galactic bulge, we exploit NuSTAR ’s ability to measure unfocused, or “zero-bounce,” X-rays from an ∼1°–3° radius field of view (FOV). The details of the NuSTAR instrument relevant for this analysis are summarized here. NuSTAR has two identical telescopes, each consisting of an independent optic and focal-plane detector, referred to as FPMA and FPMB. The optics use a conical approximation to the grazing incidence Wolter I design, in which X-rays are focused by reflection from an upper parabolic mirror section and then a lower hyperbolic mirror section. The FOV for these focused, or “two-bounce,” photons is ∼13′ × 13′. To block unfocused X-rays from reaching the NuSTAR detectors, a series of aperture stops are attached to each focal-plane bench. However, this shielding is not complete, and there remains an ∼35 radius aperture, partially blocked by the optics bench, from which totally unfocused, or zero-bounce, photons can reach the detectors. The solid-angle aperture for zero-bounce photons is over 2 orders of magnitude larger than the FOV for focused photons, even after accounting for blocking by the optics and vignetting effects due to the aperture stop. This large solid angle for zero-bounce photons allows us to study the diffuse emission of the Galactic bulge. In this analysis, we construct a spectrum using photons from the full detector plane, which are a combination of photons from the zero-bounce aperture, two-bounce FOV, and instrument background (similar to the technique used in Ng et al. 2019 ). Via this technique, we lose the imaging capabilities of the NuSTAR optics but gain spectral information from a large region of the sky. Although the aperture for zero-bounce photons is much larger, the two-bounce FOV is not entirely negligible due to the increasing intensity of the Galactic diffuse flux in the two-bounce FOV of our observations. Using the mass model of the nuclear stellar disk and bulge, as described in Section 4.3 , we estimate that the two-bounce count rate from the Galactic center in our spectrum is ∼20%–25% as high as the zero-bounce count rate from the bulge. We thus need to
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