The Galactic Bulge Diffuse Emission in Broadband 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 lowenergy (< 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 highenergy (> 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 • 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 ≈ 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.

NuSTAR tests of sterile-neutrino dark matter: New Galactic bulge observations and combined impact

“…As specified in the default NuSTAR background model, we parametrize the CXB spectrum with a cut-off power-law whose flux, spectral index, and e-folding energy are fixed to the values measured in similar energy ranges by HEAO-1 and INTEGRAL [105,106]; i.e., there are no free parameters in the CXB model. This choice is supported by a previous NuSTAR analysis using the 0-bounce technique, which obtained a CXB flux consistent with our adopted value [109]. We test the effect of allowing the CXB flux to vary by AE10% to account for cross-calibration uncertainty or the effects of cosmic variance in the ∼4.5 deg 2 FOV, as the number density of CXB sources was previously measured by NuSTAR to be ≳100 deg −2 [114].…”

“…As we exclude the E < 5 keV data, we adopt the procedure described in Refs. [33,109] and replace the apec model with a power-law. For each FPM and each obsID, we use the data collected when the telescope aperture is occulted by the Earth to constrain the power-law spectral index and normalization with respect to the internal continuum.…”

“…Broadband studies of the GRXE indicate that it is likely a multitemperature plasma, with kT 1 ≲ 1 keV and kT 2 ∼ 8 keV [107,108]. We model the GRXE, which appears in both the 0-bounce and 2-bounce FOVs, as a singletemperature collisionally-ionized plasma (the apec model described previously) with a fixed temperature of 8 keV previously measured by NuSTAR; however, this analysis was not sensitive to the elemental abundances [109]. (We are unable to leave the GRXE temperature free to fit, as we find that doing so leaves the temperature almost completely unconstrained.)…”

The ART-XC telescope on board the SRG observatory

“…In particular, we still poorly understand its composition and how it varies across the Galaxy (e.g. Koyama 2018;Perez et al 2019). Another important issue is whether truly diffuse X-ray emission, associated with the hot phase of the interstellar medium, contributes significantly to the GRXE.…”

Millisecond Pulsars from Accretion Induced Collapse as the Origin of the Galactic Centre Gamma-ray Excess Signal

“…We also need to establish whether the integrated X-ray emission of the unresolved point sources of our model is consistent with observations. Thus, we use the observed [120] 2 − 10 keV X-ray luminosity of the Galactic Bulge (6.8±1.2×10 37 erg s −1 ), shown as the data points in Figure S28 at 13.8 Gyrs. To calculate the model X-ray luminosity shown in the same figure we use moving average smoothing.…”