Abstract
Four VY Scl-type nova-like systems were observed in X-rays both during the low and the high optical states. They are BZ Cam, MV Lyr, TT Ari, and V794 Aql. Using archival ROSAT, <em>Swift</em> and <em>SUZAKU</em> observations we found that the X-ray flux for BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) that would indicate the thermonuclear burning predicted in a previous article. The X-ray flux is lower by a factor 2–10 in the low than the high state in other systems, and does not reflect the drop in <em>˙M</em> inferred from optical and UV data. The best fit model for the X-ray spectra is a collisionally ionized plasma model. The X-ray flux may originate in a shocked wind or in accretion onto polar caps in intermediate polar systems that continues even during the low state.
Highlights
Nova-like (NLs) stars are non-eruptive CVs (Warner, 1995), classified into several subtypes according to their properties
We examined the archival X-ray data of VY Scl-type stars obtained with Swift and ROSAT and chose the objects that were observed both in the high and low states: BZ Cam, MV Lyr, TT Ari and V794 Aql
TT Arietis is one of the most optically luminous CVs, usually at mag. 10–11. Sometimes it abruptly falls into an “intermediate state” around 14 mag. or even into a “low state” at about 16.5 mag
Summary
Nova-like (NLs) stars are non-eruptive CVs (Warner, 1995), classified into several subtypes according to their properties. These authors found an anti-correlation in the optical and X-ray intensity, and despite the very poor spectral resolution of the ROSAT HRI, the spectrum appeared to be very soft in the low state These authors suggested that quasi-stable thermonuclear burning occurs on the surface of the WD in the low state. In this framework, VY Scl-type stars are key objects in the evolution of interacting WD binaries, in which hydrogen burning occurs periodically without outbursts. VY Scl-type stars are key objects in the evolution of interacting WD binaries, in which hydrogen burning occurs periodically without outbursts They may reach the Chandrasekhar mass and the conditions for type Ia supernovae explosion. Using archival X-ray observations, in this paper we compare the high and low state X-ray data of 4 VY Scl-type stars attempting to reveal evidence of nuclear burning during the low state, or seeking alternative explanations for the changes that take place during the transition from the high to low state
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