title: A Comprehensive Study of Nova Persei 2018: A Gamma-ray Bright Nova from a Known Dwarf Nova
Creators: name: Murphy-Glaysher, F
Publisher: name: Liverpool John Moores University
Year: 2023

description: The eruption of a classical nova (CN) is an extremely energetic transient event that produces a rapid optical brightening of 10-15 magnitudes, followed by a slower decline in luminosity. A CN is a binary system consisting of a white dwarf (WD) primary that accretes stellar material from the less-evolved donor star. In the majority of systems, mass transfer onto the WD takes place via an accretion disk. A thermonuclear runaway is triggered when sufficient mass has accumulated on the WD, and the energy thus injected into the WD envelope causes the high velocity expulsion of the envelope in the nova eruption. Due to the rapid ejection of this shell of material, the WD photosphere expands and then contracts, which is observable as the brightening and subsequent fading of the nova light curve. A dwarf nova (DN) outburst is less luminous than a CN eruption, and occurs when material in the accretion disk is suddenly deposited onto the WD due to thermal or tidal instabilities within the disk. The corresponding release of gravitational potential energy powers the increase in luminosity. V392 Persei is a known DN that underwent a CN eruption on April 29 2018, with γ-ray emission detected from the system the following day. V392 Per provided the first opportunity to study the γ-ray emission processes in a previously studied nova system. Here we report ground-based optical, Swift UV and X-ray, and Fermi -LAT γ-ray observations following the eruption for almost three years. The optical light curve reveals that V392 Per is one of the fastest evolving novae yet observed, with a t₂ decline time of 2 days. Early spectra present evidence for multiple and interacting mass ejections, with the associated shocks driving both the γ-ray and early optical luminosity. V392 Per entered Sun constraint within days of eruption. Upon exit, the nova had evolved to the nebular phase, and we saw the tail of the super-soft X-ray phase. Subsequent optical emission captured the fading ejecta alongside a persistent narrow line emission spectrum from the accretion disk. Ongoing hard X-ray emission is characteristic of a standing accretion shock in an intermediate polar. Analysis of the optical data reveals an orbital period of 3.230 ± 0.003 days, but we see no evidence for a WD spin period. The optical and X-ray data suggest a high mass WD, the pre-nova spectral energy distribution (SED) indicates an evolved donor, and the post-nova SED points to a high mass accretion rate. Following eruption, the system has remained in a nova-like high mass transfer state, rather than returning to the pre-nova DN low mass transfer configuration. We suggest that this high state is driven by irradiation of the donor by the nova eruption. In many ways, V392 Per shows similarity to the well-studied nova and DN GK Persei. A preliminary photoionization analysis of the early nebular spectra was performed in an attempt to constrain the ionization conditions within the nova shell. Three key emission line flux ratios were measured from the spectra. The plasma simulation and spectral synthesis code cloudy was used to produce an array of models that varied the effective temperature of the WD (the ionizing source), and the electron density and metallicity of the nova shell. The measured line ratios were compared with the predicted ratios for the models. Although the results were inconclusive, they indicated some constraints on the ionization conditions that were consistent with what we might expect for a nova shell. Finally, some suggested developments of the work discussed in this thesis are presented. The first extension considered is a more complete analysis of the photoionization conditions within the shell of V392 Per, accompanied by morpho-kinematic modelling to constrain the geometry of the nova shell. Another avenue to progress this work is to conduct a further monitoring campaign on V392 Per and ascertain the ongoing mass transfer state of the system. Polarimetric observations may reveal signals of the WD magnetic field, or of a degree of dust production within the expanding shell. Perhaps the most exciting possibility would be to apply the same analytical techniques to observations of a system similar to V392 Per, but which does not experience Sun constraint at such an early stage of its evolution.