Abstract Fermi observations have revealed excess γ -ray emission close to the Galactic Center, which could be produced by dark matter annihilation or by unresolved millisecond pulsars (MSPs). We use archival XMM Newton and Chandra observations of the Galactic Bulge to search for possible MSPs, with the goal of eventually testing the MSP origin hypothesis for the γ -ray excess. We compare the X-rays in the 6.2–7.2 keV energy range with emission in neighboring energy ranges, 5.8–6.2 keV and 7.2–7.6 keV, to search for fluorescent Fe and Fe-K lines in X-ray spectra. We thus create an X-ray color to distinguish continuum sources from line-dominated sources, like cataclysmic variables (CVs). We find 54 XMM Newton sources and 83 Chandra sources that do not show evidence (either photometric, or via spectral fitting) for Fe lines, among 185 XMM-Newton sources and 288 Chandra sources with ≥250 counts allowing strong tests for lines. These sources lacking Fe lines include known X-ray binaries, pulsar wind nebulae, and a magnetar. Thirteen sources lacking Fe lines of unknown nature appear to have radio counterparts. These sources without evidence for Fe lines likely contain many X-ray binaries, MSPs, some background active galactic nuclei (∼34 XMM-Newton sources and ∼5 Chandra sources), and possible CVs with very faint Fe emission lines (∼15 XMM-Newton sources and ∼17 Chandra sources). We suggest that detailed follow-up of these unidentified X-ray sources lacking lines may uncover the brightest of the expected population of bulge MSPs.
Hebbar et al. (Wed,) studied this question.