Multiwavelength Observations of Multiple Eruptions of the Recurrent Nova M31N 2008-12a

We report the optical, UV, and soft X-ray observations of the 2017–2022 eruptions of the recurrent nova M31N 2008-12a. We find a cusp feature in the $r^{\prime} $ - and $i^{\prime} $ -band light curves close to the peak, which could be related to jets. The geometry of the nova ejecta based on morpho-kinematic modeling of the Hα emission line indicates an extended jet-like bipolar structure. Spectral modeling indicates an ejecta mass of 10^‑7–10^‑8 M _⊙ during each eruption and an enhanced helium abundance. The supersoft source phase shows significant variability, which is anticorrelated to the UV emission, indicating a common origin. The variability could be due to the reformation of the accretion disk. We infer a steady decrease in the accretion rate over the years based on the intereruption recurrence period. A comparison of the accretion rate with different models on the ${M}_{\mathrm{WD}}\mbox{--}\dot{M}$ plane yields the mass of a CO white dwarf, powering the H-shell flashes every ∼1 yr, to be >1.36 M _⊙ and growing with time, making M31N 2008-12a a strong candidate for the single degenerate scenario of the Type Ia supernovae progenitor.