Challenging classical paradigms: Recurrent nova M31N 2017-01e, a BeWD system in M31?

Abstract

M31N 2017-01e is the second-fastest recurrent nova known, with a recurrence period of 2.5 yr in the Andromeda galaxy (M31). This system exhibits a unique combination of properties: a low outburst amplitude (∼3 mag), starkly contrasting with known recurrent novae (typically ≥6 mag), and a very fast evolution (t2 ∼ 5 days). Its position coincides with a bright variable source (MV ∼ ‑4.2, B ‑ V = 0.042) displaying a 14.3 day photometric modulation, which has been suggested as the likely progenitor. We present a multiwavelength analysis of optical and UV data spanning quiescence and the 2019 and 2024 outbursts. Archival high-resolution imaging reveals two nearby faint sources within 5″ of the proposed nova system, which we identified as unrelated field stars. Color analysis and spectral energy distribution fitting suggest the progenitor is likely an early-type star. Combined with archival spectra consistent with a B-type star with Hα in emission, this points to the quiescent counterpart being a Be star with a circumstellar disk. We propose that M31N 2017-01e arises from a rare BeWD binary, where the white dwarf (WD) accretes from the decretion disk of its companion, explaining its rapid recurrence, low-amplitude outbursts, and unusual quiescent luminosity and color. This analysis highlights M31N 2017-01e as a compelling outlier among recurrent novae, suggesting a distinct accretion mechanism and evolutionary path that challenges the prevailing paradigm.