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.Comment: 2 figures, emulateapj, to appear in ApJ
