X-ray Spectra and Light Curves of Cooling Novae and a Nova-Like

TL;DR

This study presents X-ray observations of various novae and related systems, revealing differences in accretion rates, magnetic properties, and variability, with implications for understanding their post-outburst evolution and accretion behaviors.

Contribution

It provides detailed X-ray spectral and timing analysis of several novae and nova-like systems, identifying magnetic signatures and accretion rates, and highlighting variability in X-ray and optical emissions.

Findings

V2491 Cyg shows signs of an intermediate polar with a ~39 min period.

KT Eri has a lower accretion rate and different X-ray characteristics.

V794 Aql exhibits unstable accretion rates with no clear optical-X-ray correlation.

Abstract

We present X-ray observations of novae V2491 Cyg and KT Eri about 9 years post-outburst, of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the Chandra ACIS-S camera, V794 Aql with the Chandra ACIS-S camera and High Energy Transmission Gratings. The two recent novae, similar in outburst amplitude and light curve, appear very different at quiescence. Assuming half of the gravitational energy is irradiated in X-rays, V2491 Cyg is accreting at $\dot{m}=1.4\times10^{-9}-10^{-8}M_\odot/yr$, while for KT Eri, $\dot{m}<2\times10^{-10}M_\odot/yr$. V2491 Cyg shows signatures of a magnetized WD, specifically of an intermediate polar. A periodicity of ~39 minutes, detected in outburst, was still measured and is likely due to WD rotation. EY Cyg is accreting at $\dot{m}\sim1.8\times10^{-11}M_\odot/yr$, one magnitude lower than KT Eri, consistently with its U Gem outburst behavior and its quiescent UV flux. The X-rays are modulated with the orbital period, despite the system's low inclination, probably due to the X-ray flux of the secondary. A period of ~81 minutes is also detected, suggesting that it may also be an intermediate polar. V794 Aql had low X-ray luminosity during an optically high state, about the same level as in a recent optically low state. Thus, we find no clear correlation between optical and X-ray luminosity: the accretion rate seems unstable and variable. The very hard X-ray spectrum indicates a massive WD.