Multiple flares caused by mass ejection episodes during the advanced nebular phase of Nova Scuti 2019

Abstract

ABSTRACT Our photometric and spectroscopic monitoring shows that starting with 2020 June 4, day  +217 from optical maximum and well into its advanced nebular stage, Nova Scuti (Nova Sct) 2019 begun displaying a series of nine large amplitude flares (up to $\Delta m \sim 1.7$ mag), characterized by a rapid rise to peak (≤10 h) and a fast exponential decline (e-folding time ∼50 h). The time interval Δt between flares follows an ordered sequence, declining from 8.43 to 4.90 d, that safely allows to exclude that any other flare occurred without being recorded by the observations. When the sequence of flares was over by 2020 July 28 (day  +271), Nova Sct 2019 slowed its overall decline rate from Δm = 0.0067 to 0.0027 mag d−1. The flares were caused by material expelled at high velocity (∼1000 km s−1) from the still burning white dwarf (WD). The cooler pseudo-photosphere forming at each flare in the expelled material, resulted in a recombination wave to spread through the original nova ejecta (at ∼170 au from the WD), quenching emission from [Fe x] and [Fe vii] and boosting that from lower ionization species. After each flare, once the small amount of expelled material had turned optically thin, the original nova ejecta resumed displaying [Fe x] and [Fe vii] emission lines, a fact that clearly proves the direct photoionization action exerted on the ejecta by the burning WD. While the other known flaring novae (V458 Vul, V4745 Sgr, and V5588 Sgr) presented the flares close to maximum brightness and with increasing Δt, Nova Sct 2019 is unique in having displayed them during the advanced nebular stage and with decreasing Δt.