X-ray observations of Postoutburst Nova V5116 Sagittarii
14 March 2008
In a recent paper, Gloria Sala et al. present the X-ray light curve and broadband spectrum of nova V5116 Sagittarii (V5116 Sgr) as observed by XMM-Newton 609 days after the nova was first observed at optical wavelengths.
When a white dwarf star in a binary system accretes material from the larger companion star and the accreted gas after reaching a critical limit explosively ignites as hydrogen burning kicks in, a nova results. This nova explosion causes part of the outer gas material to be flung into space and the white dwarf to brighten by several magnitudes.
Nova V5116 Sgr (R.A. 18h17m50.80s, Dec. -30°26'32.4" [J2000.0]) was discovered in optical groundbased observations by W. Liller on 4 July 2005 (IAU Circular 8559) when it appeared as an 8th magnitude star - and one day later peaking at 7.2 - where earlier observations revealed no object down to 11th magnitude.
Follow up observations of these type of objects in X-rays can provide detailed information on the nature of the postoutburst nova, as after the initial burst in optical emission the expelled material becomes transparent to soft X-ray emission coming from hydrogen burning in the remnant shell of gas around the white dwarf.
On 5 March 2007, XMM-Newton observed V5116 Sgr for a period of over three hours and obtained the soft X-ray lightcurve and broadband spectrum. The results are presented by G. Sala et al. and show the postoutburst nova to have no significant emission above 1 keV (EPIC is sensitive up to 15 keV) with the spectrum peaking between 0.4-0.5 keV.
The lightcurve of the emission in the energy range 0.2-1 keV shows a low state during which there are no significant variations, and a high state where the emission is a factor of about 8 stronger and flaring activity is observed.
Figure 1: Soft X-ray lightcurve of V5116 Sgr obtained with the EPIC instrument on 5 March covering the energy range 0.2-1.0 keV.
Credit: G. Sala et al. 
Although the observation period is too short to be conclusive, the time between the two ends of the observed two high states (2.95±0.03 h) is consistent with the periodicity observed in the nova's optical emission (2.9712±0.0024 h) that is taken to be the orbital period of the binary system.
Figure 2: Soft X-ray spectrum of V5116 Sgr during the low state and high state over the energy range 0.2-0.7 keV, plotted with the fitted ONe white dwarf atmosphere model.
Credit: G. Sala et al. 
The EPIC spectral data shows no significant changes in the white dwarf's spectrum over the whole observation period. The spectra were compared with different white dwarf models and were found to be only compatible with an oxygen/neon (ONe) white dwarf model, ruling out the carbon/oxygen models.
The atmosphere temperature of the fitted ONe model was found to be 6.1±0.1 × 105 K, both for the low state and high state. This means that the factor ~8 difference in observed flux between the high and low state is not intrinsic to the hydrogen burning on top of the white dwarf, but rather is due to an obscuring agent.
G. Sala et al. propose that the observed dimming of the white dwarf's soft X-ray emission is due to an asymmetric accretion disk of material from the companion star, partially obscuring the white dwarf for ~72% of the time every orbital period of the binary system.
Sala, G.; Hernanz, M.; Ferri, C.; Greiner, J., "V5116 Sagittarii, an Eclipsing Supersoft Postoutburst Nova?", ApJ. 675, L93-L96, DOI: 10.1086/533530, 10 Mar 2008
Last Update: 31 March 2008