The Broad-band View of the Galactic Bulge X-ray Population
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 $\sim100$ 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 $\sim$1--3$^\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.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1255P