The Galactic Bulge Diffuse Emission in Broad-Band X-rays with NuSTAR

Abstract: The diffuse hard X-ray emission that fills the Galactic center, bulge, and ridge is believed to arise from unresolved populations of X-ray binary systems. However, the identity of the dominant class of accreting objects in each region remains unclear. Recent studies of Fe line properties and the low-energy (<10 keV) X-ray continuum of the bulge indicate a major population fraction of non-magnetic cataclysmic variables (CVs), in particular quiescent dwarf novae. This is in contrast to previous high-energy (>10 keV) X-ray measurements of the bulge and ridge, which indicate a dominant population of magnetic CVs, in particular intermediate polars. In addition, NuSTAR broad-band measurements have uncovered a much heavier intermediate polar population in the central ~100 pc than previously assumed, raising the possibility that some fraction of this population extends further from the center. Here we use NuSTAR's large aperture for unfocused photons and its broad-band X-ray range to probe the diffuse continuum of the inner ~1-3$^\circ$ of the Galactic bulge. This allows us to constrain possible multi-temperature components of the spectrum, such as could indicate a mixture of soft and hard populations. Our emissivity is consistent with previous hard X-ray measurements in the bulge and ridge, with the diffuse X-ray luminosity tracing the stellar mass. The spectrum is well-described by a single-temperature thermal plasma with $kT \approx 8$ keV, with no significant emission above 20 keV. This supports that the bulge is dominated by quiescent dwarf novae; we find no evidence of a significant intermediate polar population in the hard X-ray band.