X-ray Spectroscopy of the Dwarf Nova Z Chamaeleontis in Quiescence and Outburst Using the XMM-Newton Observatory

Abstract

We present X-ray spectroscopy of the SU UMa-type dwarf nova (DN) Z Cha using the EPIC and RGS instruments onboard the XMM-Newton Observatory. The quiescent system can be modeled by collisional equilibrium or nonequilibrium plasma models, yielding a kT of 8.2-13.0 keV at a luminosity of (5.0-6.0)$\times$10${}^{30}$ erg/s. The spectra yield better reduced ${\chi }^2$ using partial covering absorbers of cold and photoionized nature. The ionized absorber has an equivalent N${}_H$=(3.4-5.9)$\times$10${}^{22}$ cm${}^{-2}$ and a log($\xi$)=3.5-3.7 with (50-60)% covering fraction when VNEI model (XSPEC) is used. The line diagnosis in quiescence shows no resonance lines with only detected forbidden lines of Ne, Mg, Si. The H-like C, O, Ne, and Mg are detected. The strongest line is O VIII with (2.7-4.6)$\times$10${}^{-14}$ erg/s/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 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 yet not in a collisional equilibrium, revealing ionization timescales of (0.97-1.4)$\times$10${}^{11}$ s cm${}^{-3}$. The R-ratios in outburst yield electron densities of (7-90)$\times$10${}^{11}$ cm${}^{-3}$ and the G-ratios yield electron temperatures of (2-3)$\times$10${}^6$ K. The outburst luminosity is (1.4-2.5)$\times$10${}^{30}$ erg/s. 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. This is too low for boundary layers, consistent with the nature of ADAF-like hot flows.

27 Pages, 8 Figures, and 5 Tables, accepted to be published in the Astrophysical Journal, in press