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.

Figures (10)

• Figure 1: Colour and light curves of Nova Vul 2024. The Strömgren $y$-band magnitude and the H$\alpha$ emission-line flux, expressed in magnitudes, are obtained by integrating the flux-calibrated spectra. Black dots are ZTF $g$ magnitudes rescaled to AAVSO $V$ magnitudes by adding a $-$1 mag offset. The observing epochs are counted as days passed since optical maximum (2024 Jul 30.8 UT). The green ticks in the $V$-band panel mark the epoch of Echelle observations, while the blue ticks are the epochs of low resolution spectra. The arrows point to epochs of spectra shown in Fig. \ref{['oscillations']} that were obtained at minima and maxima during oscillation period.
• Figure 2: Log-log plot of the time evolution of the flux radiated by NVul24 through the $V$ band. The fit with a broken power-law of the form $F^{V} \propto (t-t_0)^{\alpha}$ is overplotted (lines in black) and the corresponding values of $\alpha$ are quoted. Red dots are from AAVSO magnitudes, binned each half-day. Orange dots are from ZTF $g$ magnitudes rescaled to $V$ magnitudes by adding a $-$1 mag offset.
• Figure 3: PanSTARRS $r$ image cropped around the position of NVul24. The size is 30x30 arcsec. The red circle has a radius of 1.5 arcsec, and it is centred at the accurate position we measured for NVul24 (cf. sect. \ref{['sec:astrometry']}). The star close to the centre of the circle is GAIA DR3 1825912166611947136 (of $G$=19.82 mag), which possibility to be the progenitor of the nova is discussed in sect. \ref{['sec:astrometry']}.
• Figure 4: Spectral evolution of NVul24 for the first 50 days of the outburst. The sequence of sample spectra were obtained in low dispersion with the Asiago 1.22m+B&C (day +11, +16, +24) and in high resolution with the Varese 0.84m+ Echelle (day +0, +1, +2, +4, +7, +34, +42). The latter were Gaussian smoothed and binned to reduce their spectral resolution to match that of the Asiago 1.22m+B&C spectra, for an easier intercomparison. The native absolute fluxing of all the spectra has been checked against the AAVSO photometry of Fig. \ref{['phot1']}, and dereddened by $E_{B-V}$=1.6. The offset in flux applied to the spectra for a clearer plotting is indicated in green.
• Figure 5: Evolution of H$\alpha$, and OI 8446 Å emission lines. The H$\alpha$ line profile has been corrected for telluric absorbptions. No correction was applied to OI 8446 Å line because telluric absorptions ends redward 8377 Å . The drop in intensity between 8555 and 8575 Å is artificial and represents an inter-order gap in the Echelle spectra.