Persistent nuclear burning in Nova Sgr 2016 N.4 (=V5856 Sgr = ASASSN-16ma) six years past its outburst

Abstract

We report on the fast Nova Sgr 2016 N.4 being surprisingly trapped in a long-lasting and bright plateau (ΔI≥10 mag above quiescence) six years past the nova eruption. Very few other novae experience a similar occurrence. We carried out an intensive observing campaign collecting dailyBVRIphotometry and monthly high-resolution optical spectroscopy, and observed the nova in ultraviolet and X-rays withSwiftat five distinct epochs. The bolometric luminosity radiated during the plateau is ∼4200L⊙(scaled to the distance of the Galactic Bulge), corresponding to stable nuclear burning on a 0.6M⊙white dwarf. A stable wind is blown off at full width at zero intensity (FWZI) ∼ 1600 km s−1, with episodic reinforcement of a faster FWZI ∼ 3400 km s−1mass loss, probably oriented along the polar directions. The collision of these winds could power the emission detected in X-rays. The burning shell has an outer radius of ∼25R⊙at which the effective temperature is ∼7600 K, values similar to those of a F0 II/Ib bright giant. The Δm < 1 mag variability displayed during the plateau is best described as chaotic, with the irregular appearance of quasi-periodic oscillations with a periodicity of 15–17 days. A limited amount of dust (≈3 × 10−11M⊙) continuously condenses atTdust ∼ 1200 K in the outflowing wind, radiatingLdust ∼ 52L⊙.