V5116 Sgr, an Eclipsing Supersoft Post-Outburst Nova?

TL;DR

This study analyzes XMM-Newton observations of V5116 Sgr, revealing it as an eclipsing supersoft X-ray source with flux variability likely caused by an asymmetric accretion disk, and confirms residual hydrogen burning on the white dwarf.

Contribution

It provides the first detailed X-ray spectral analysis of V5116 Sgr post-outburst, identifying the white dwarf atmosphere and proposing a new explanation for flux variability.

Findings

V5116 Sgr is an eclipsing supersoft X-ray source.

White dwarf atmosphere models fit the spectra well.

Flux variability is likely due to an asymmetric accretion disk.

Abstract

V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light-curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 h, the orbital period suggested by Dobrotka et al. (2007), although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11+/-3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with reduced chi^2>4. In contrast, ONe rich white dwarf atmosphere models provide a good fit, with nH=1.3(+/-0.1)e21 cm^-2, T=6.1(+/-0.1)e5 K, and L=3.9(+/-0.8)e37(D/10kpc)^2 erg/s (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low and the high flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light-curve may result from a partial coverage by an asymmetric accretion disk in a high inclination system.