Characterizing the Advective Hot Flows of Nova-Like Cataclysmic Variables in the X-rays: The case of BZ Cam and V592 Cas

Abstract

We present a joint spectral analysis of ROSAT PSPC, Swift XRT, and NuSTAR FPMA/B data of the nova-like (NL) cataclysmic variables (CVs), BZ Cam and V592 Cas in the 0.1-78.0 keV band. Plasma models of collisional equilibrium fail to model the 6.0-7.0 iron line complex and continuum with $\chi^2_\nu$ larger than 2.0. Our results show nonequilibrium ionization (NEI) conditions in the X-ray plasma with temperatures of 8.2-9.4 keV and 10.0-12.9 keV for BZ Cam and V592 Cas, respectively. The centroids of He-like and H-like iron ionization lines are not at their equilibrium values as expected from NEI conditions. We find power law spectral components that reveal the existence of scattering and Comptonization with a photon index of 1.50-1.87. We detect a P Cygni profile in the H-like iron line of BZ Cam translating to outflows of 4500-8700 km s$^{-1}$ consistent with the fast winds in the optical and UV. This is the first time such a fast collimated outflow is detected in the X-rays from an accreting CV. An Iron K$\alpha$ line around 6.2-6.5 keV is found revealing the existence of reflection effects in both sources. We study the broadband noise and find that the optically thick disk truncates in BZ Cam and V592 Cas consistent with transition to an advective hot flow structure. V592 Cas also exhibits a quasi-periodic oscillation at 1.4$^{+2.6}_{-0.3}$ mHz. In general, we find that the two NLs portray spectral and noise characteristics as expected from advective hot accretion flows at low radiative efficiency.