SWIFT XRT Observations of the Nova-like Cataclysmic Variables MV Lyr, BZ Cam and V592 Cas

TL;DR

This study uses SWIFT XRT observations to analyze the boundary layer characteristics of three nova-like cataclysmic variables, revealing high-temperature, optically thin emission and possible ADAF-like flows, with implications for understanding their accretion processes.

Contribution

First detailed X-ray spectral analysis of boundary layers in non-magnetic nova-like CVs, suggesting ADAF-like flows and wind outflows as key features.

Findings

Boundary layers exhibit high virial temperatures (21-50 keV).

Detection of power-law components indicating Compton scattering effects.

X-ray luminosities are a small fraction (~0.001-0.01) of disk luminosities.

Abstract

We present a total of ~ 45 ksec (3$\times$15 ksec) of SWIFT XRT observations for three non-magnetic nova-like (NL) Cataclysmic Variables (CVs) (MV Lyr, BZ Cam, V592 Cas) in order to study characteristics of Boundary Layers (BL) in CVs. The nonmagnetic NLs are found mostly in a state of high mass accretion rate ($\ge$1$\times$10$^{-9}$ Msun yr$^{-1}$) and some show occasional low states. Using the XRT data, we find optically thin multiple-temperature cooling flow type emission spectra with X-ray temperatures (kT$_{max}$) of 21-50 keV. These hard X-ray emitting boundary layers diverge from simple isobaric cooling flows indicating X-ray temperatures that are of virial values in the disk. In addition, we detect power law emission components from MV Lyr and BZ Cam and plausibly from V592 Cas which may be a result of the Compton scattering of the optically thin emission from the fast wind outflows in these systems and/or Compton up-scattering of the soft disk photons. The X-ray luminosities of the (multi-temperature) thermal plasma emission in the 0.1-50.0 keV range are (0.9-5.0)$\times$10$^{32}$ erg/sec. The ratio of the X-ray and disk luminosities (calculated from the UV-optical wavelengths) yield an efficiency (L$_{x}$/L$_{disk}$) ~ 0.01-0.001. Given this non-radiative ratio for the X-ray emitting boundary layers with no significant optically thick blackbody emission in the soft X-rays (consistent with ROSAT observations) together with the high/virial X-ray temperatures, we suggest that high state NL systems may have optically thin BLs merged with ADAF-like flows and/or X-ray coronae. In addition, we note that the axisymmetric bipolar and/or rotation dominated fast wind outflows detected in these three NLs (particularly BZ Cam and V592 Cas) or some other NL may also be explained in the context of ADAF-like BL regions.