The hybrid Nova Vul 2024 (=V615 Vul)

Abstract: We present daily optical, absolute-fluxed spectroscopy of the heavily reddened Nova Vul 2024 (=V615 Vul), obtained in both low- and high-resolution echelle modes from discovery on 29 July 2024 through the nebular phase. Refined astrometry confirms the positional consistency with a potential progenitor. Reliable estimates of the decline time (t3=10.7 +/- 0.5 d) and reddening (E(B-V)=1.6 +/- 0.1) enabled the application of the Maximum Magnitude - Rate of Decline relation, yielding a distance of 5.0 +/- 1.0 kpc in excellent agreement with 3D Galactic extinction maps. Around maximum light, the nova displayed a Fe II-type spectrum with very broad emission lines (FWZI 5800 km s-1) and high-velocity P-Cygni absorptions. Shortly after t3, coincident with the onset of hard X-ray emission, it showed pronounced photometric oscillations driven mainly by continuum variations. At the same time, He/N features developed alongside Fe II lines, identifying V615 Vul as a rare hybrid nova. During the nebular phase, the ionization level increased up to [Fe VII] and likely [Fe X]. The ejecta show no evidence of neon overabundance and expand ballistically, as indicated by the constant line widths, stable profiles, and characteristic castellation of the high-resolution emission lines.