Catching a nova X-ray/UV flash in the visible? Early spectroscopy of the very slow Nova Velorum 2022 (Gaia22alz)

Abstract

We present early spectral observations of the very slow Galactic nova Gaia22alz, over its gradual rise to peak brightness that lasted 180 days. During the first 50 days, when the nova was only 3–4 magnitudes above its normal brightness, the spectra showed narrow (FWHM ≈ 400 km s−1) emission lines of H Balmer, He i, He ii, and C iv, but no P Cygni absorption. A few weeks later, the high-excitation He ii and C iv lines disappeared, and P Cygni profiles of Balmer, He i, and eventually Fe ii lines emerged, yielding a spectrum typical of classical novae before peak. We propose that the early (first 50 days) spectra of Gaia22alz, particularly the emission lines with no P Cygni profiles, are produced in the white dwarf’s optically thin envelope or accretion disc, reprocessing ultraviolet and potentially X-ray emission from the white dwarf after a dramatic increase in the rate of thermonuclear reactions, during a phase known as the ‘early X-ray/UV flash’. If true, this would be one of the rare times that the optical signature of the early X-ray/UV flash has been detected. While this phase might last only a few hours in other novae and thus be easily missed, it was possible to detect in Gaia22alz due to its very slow and gradual rise and thanks to the efficiency of new all-sky surveys in detecting transients on their rise. We also consider alternative scenarios that could explain the early spectral features of Gaia22alz and its gradual rise.

Hydrodynamic Simulations of Oxygen–Neon Classical Novae as Galactic 7Li Producers and Potential Accretion-induced Collapse Progenitors*

“…8. Although our simulations using a composition of 25% WD matter and 75% solar matter (MDTNR) on low-mass WDs, either CO or ONe, do not appear to resemble the outbursts of typical CNe, they are sufficiently slow to resemble the recent outburst of Nova Velorum 2022 (Aydi et al 2023) and we predict that the WD in this system has a very low mass. 9.…”

“…Peak M bol is an increasing function of WD mass but even the simulation of the most massive WD does not reach a value that is observed in a typical CN outburst. The simulations of low-mass WDs might fit some of the slowest and faintest CNe shown in Kasliwal et al (2011) or the evolution of Nova Velorum 2022 as described by Aydi et al (2023). Obtaining the element abundances for the Nova Velorum 2022 ejecta could test these simulations.…”

“…The simulation for the 0.6 M e WD evolves so slowly, however, that it takes almost 8000 s for the temperature to reach its peak and the decline is even slower. In fact, the evolution of the TNR on this low-mass WD is so slow that it rules out this WD mass for all but the slowest CNe (either CO or ONe) such as Nova Velorum 2022 (Aydi et al 2023).…”

Spectra of V1405 Cas at the Very Beginning Indicate a Low-mass ONeMg White Dwarf Progenitor

“…It is likely that these high-ionization lines are only strong in the short timescale just after the nova eruption; such very early-phase spectra are largely missing in the literature. Another important object in this context is the extremely slow nova Gaia22alz (Brink et al 2022;Aydi et al 2023); its first spectrum, which was taken when it was only 4 magnitude above its quiescent, resembled that of T Pyx in the existence of prominent Balmer, He I, He II, N III, and C IV lines.…”

“…Therefore, spectroscopic observations in such an infant phase are expected to provide new insight into the natures of the WD progenitor and the ejecta dynamics just after the initiation of the thermonuclear runaway. However, it is an extremely challenging observation, given the short timescale associated with the initial brightening; except for some symbiotic novae or extremely slow novae, e.g., Gaia22alz (Brink et al 2022;Aydi et al 2023), only a single example, the recurrent nova T Pyx in its 2011 outburst, has been reported for such a prompt spectroscopic observation (Arai et al 2015).…”