2024ApJ...977..136B

We present X-ray spectroscopy of the SU UMa-type dwarf nova Z Chamaeleontis using the European Photon Imaging Camera and reflection grating spectrometer (RGS) instruments on board the XMM-Newton observatory. The quiescent system can be modeled by collisional equilibrium (CIE) or nonequilibrium plasma models, yielding a kT of 8.2-13.0 keV at a luminosity of (5.0-6.0) × 10³⁰ erg s^–1. The spectra yield better ${\chi }_{\nu }^{2}$ using partially covering absorbers of cold and/or photoionized nature. The ionized absorber has an equivalent N_H = (3.4-5.9) × 10²² cm^–2 and a log(ξ) = 3.5-3.7 with a (50-60)% covering fraction when the VNEI model (XSPEC) is used. The line diagnosis in quiescence shows no resonance lines, with only the forbidden lines of Ne, Mg, and Si detected. The H-like C, O, Ne, and Mg are detected. The strongest line is O VIII, with (2.7-4.6) × 10^–14 erg s^–1 cm^–2. The quiescent X-ray-emitting plasma is not collisional and not in ionization equilibrium, which is consistent with hot ADAF-like accretion flows. The line diagnosis in the outburst shows He-like O and Ne, with intercombination lines being the strongest, along with weaker resonance lines. This indicates the plasma is more collisional and denser, but not yet in CIE, revealing ionization timescales of (0.97-1.4) × 10¹¹ s cm^–3. The R ratios in the outburst yield electron densities of (7-90) × 10¹¹ cm^–3 and the G ratios yield electron temperatures of (2-3) × 10⁶ K. The outburst luminosity is (1.4-2.5) × 10³⁰ erg s^–1. The flow is inhomogeneous in density. All detected lines are narrow, with widths limited by the resolution of RGS, yielding Keplerian rotational velocities <1000 km s^–1. This is too low for boundary layers, consistent with the nature of ADAF-like hot flows.