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 years in the Andromeda Galaxy (M31). This system exhibits a unique combination of properties: a low outburst amplitude ($\sim3$ magnitude), starkly contrasting with known recurrent novae (typically $\geq 6$ magnitudes), and a very fast evolution ($t_{2}\sim 5 $ days). Its position coincides with a bright variable source ($\mathrm{{M_V \sim -4.2,\, B-V= 0.042}}$) displaying a 14.3 day photometric modulation, which has been suggested as the likely progenitor. We present a multi-wavelength analysis of optical and UV data spanning quiescence and the 2019 and 2024 outbursts. Archival high-resolution imaging reveals two nearby faint sources within $5^{\prime\prime}$ 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 H$\alpha$ in emission, this points to the quiescent counterpart being a Be star with a circumstellar disc. We propose that M31N 2017-01e arises from a rare Be-WD binary, where the 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.