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

¹ INAF-Osservatorio Astronomico di Padova, 36012 Asiago (VI), Italy
e-mail: ulisse.munari@inaf.it
² INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 101, 40129 Bologna, Italy
³ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁴ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ ANS Collaboration, c/o Astronomical Observatory, 36012 Asiago (VI), Italy

Received: 14 July 2022
Accepted: 3 August 2022

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 daily BVRI photometry and monthly high-resolution optical spectroscopy, and observed the nova in ultraviolet and X-rays with Swift at five distinct epochs. The bolometric luminosity radiated during the plateau is ∼4200 L_⊙ (scaled to the distance of the Galactic Bulge), corresponding to stable nuclear burning on a 0.6 M_⊙ 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⁻¹ mass 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 ∼25 R_⊙ 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 × 10⁻¹¹ M_⊙) continuously condenses at T_dust ∼ 1200 K in the outflowing wind, radiating L_dust ∼ 52 L_⊙.