Massive White Dwarfs in the Galactic Center: A Chandra X-Ray Spectroscopy of Cataclysmic Variables

Abstract

Abstract Previous X-ray observations toward the nuclear star cluster (NSC) at the Galactic center have discovered thousands of point sources, most of which were believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting from a low-mass companion. However, the population properties of these CVs remain unclear, which otherwise would provide important information about the evolutionary history of the NSC. In this work we utilize ultra-deep archival Chandra observations to study the spectral properties of the NSC CVs, in close comparison with those in the solar vicinity. We find that the NSC CVs have strong Fe xxv and Fe xxvi lines (both of which show equivalent widths ∼200–300 eV), indicating metal-rich companions. Moreover, their Fe xxvi to Fe xxv line flux ratio is used to diagnose the characteristic WD mass (M WD) of NSC CVs. The results show that the CVs with erg s−1 have a mean M WD of ∼0.6/1.0 M ⊙ if they are magnetic/nonmagnetic CVs; while those with between 1 and 6 × 1031 erg s−1 have a mean M WD of ∼0.8/1.2 M ⊙ if they are magnetic/nonmagnetic CVs. All these Chandra detected CVs collectively contribute ∼30%–50% of the unresolved 20–40 keV X-ray emission from the NSC. The CV population with massive (i.e., M WD ∼ 1.2 M ⊙) WDs have not been observed in the solar vicinity or the Galactic bulge, and they might have been formed via dynamical encounters in the NSC.