The flux ratio of Fe XXVI-Lyα to Fe XXV-Heα lines (I 7.0 /I 6.7 ) is a sensitive indicator of the maximum temperature (T max ), and therefore the mass of white dwarf stars (M WD ) in cataclysmic variables (CVs). To examine and calibrate the theoretical I 7.0 /I 6.7 -T max -M WD relations, reliable measurements of T max and I 7.0 /I 6.7 are necessary. In this work, we conduct a thorough investigation on 3-50 keV X-ray spectra of 25 solar neighborhood magnetic and non-magnetic CVs based on archivalNuSTAR and Suzaku observations. The measured T max are compared to the I 7.0 /I 6.7 and M WD . The results show the sampled CVs closely follow the theoretical I 7.0 /I 6.7 -T max relation. Moreover, all the M WD estimated from I 7.0 /I 6.7 are consistent with the dynamically measured ones. We conclude that I 7.0 /I 6.7 can be used as a good diagnostic for T max and M WD in both magnetic and non-magnetic CVs.

The mean white dwarf (WD) mass in the Galactic bulge cataclysmic variables (CVs) was measured by applying the shock temperature-WD mass correlation of magnetic cataclysmic variables (mCVs) to the Galactic bulge X-ray emission (GBXE) spectra. However, the resulting mean WD mass is lower than that of the local CVs. This discrepancy could be explained by the dominating sources in the GBXE, which are non-mCVs instead of mCVs. In this work, we conduct a thorough investigation of the X-ray spectra of local DNe from the Suzaku archives and derive semi-empirical correlations between the shock temperature T max, the flux ratio of Fe xxvi–Lyα to Fe xxv–Heα lines, and WD mass for quiescent, nonmagnetic CVs. By applying these correlations to the GBXE, we derive the average WD mass of CVs in the Galactic bulge to be 0.81 ± 0.07M ⊙. This value is consistent with previous optical measurements of WD mass in local CVs.

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 contain important information about the evolutionary history of the NSC. In this work we utilize ultradeep 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 white dwarf mass (M WD ) of NSC CVs. The results show that the CVs with L 2−10keV > 6 × 10 31 erg s −1 have a mean M WD of ∼ 0.6/1.0 M ⊙ if they are magnetic/non-magnetic CVs; while those with L 2−10keV between 1 − 6 × 10 31 erg s −1 have a mean xuxj@nju.edu.cn M WD of ∼ 0.8/1.2 M ⊙ if they are magnetic/non-magnetic 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 ∼ 1.2M ⊙ ) 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.

In this work we conduct a thorough investigation of the X-ray and ultraviolet (UV) properties of Y Gem based on six archival XMM-Newton and Chandra observations to explore the nature of the system. The results show that Y Gem has strong (1032–34 erg s−1) X-ray emission, including a hard (with a maximum emission temperature of 8–16 keV) and a soft (with emission temperatures of 0.02–0.2 and 0.2–0.9 keV) component. The integrated UV luminosity of Y Gem reaches ∼1035 erg s−1. We show that the previous asymptotic giant branch-main-sequence (AGB-MS) Roche-lobe overflow (RLOF) scenario is dynamically unstable and can hardly explain the ∼10 keV X-ray emission temperature. We propose Y Gem as a symbiotic star, where a white dwarf (WD) accretes from its AGB companion based on its X-ray and UV properties. We make numerical simulations to examine the evolutionary history of this system. The simulations can produce the observed properties of Y Gem in the wind WRLOF scenario. An ∼0.8M ⊙ WD with a ∼1.0–1.8M ⊙ companion in a ∼2000–32,000 day initial orbit may evolve to a Y Gem-like system. Our finding implies a potential population of symbiotic stars that may have been misclassified as AGB-MS binaries. What is more, their high mass accretion rates may enable mass accumulation to the WD and makes them candidates of Type Ia supernovae progenitors.

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