Data-Driven Constraints on Magnetar Population: No Evidence for a Distinct White Dwarf Channel

Abstract

Magnetars are usually interpreted as highly magnetized neutron stars, yet a small subset of low spin-down sources has motivated alternative scenarios involving highly magnetized white dwarfs. We test whether the observed magnetar sample is consistent with a single neutron-star population or whether the data favor an additional compact-object channel. We combine exploratory machine-learning diagnostics with hierarchical Bayesian population modeling. First, we apply K-means clustering and principal component analysis in a five-dimensional feature space $(P,\dot{P},L_X,kT,|Z|)$, where $P$ is the spin period, $\dot{P}$ its time derivative, $L_X$ the X-ray luminosity, $kT$ the thermal spectral temperature, and $|Z|$ the absolute Galactic scale height. We then train a Random Forest classifier with leave-one-out cross-validation to identify the observables driving the empirical split. Subsequently, we construct a hierarchical Bayesian mixture model linking spin parameters to magnetic-field distributions through covariate-dependent mixing fractions. Posterior inference is performed with Hamiltonian Monte Carlo, and predictive performance is assessed using Pareto-smoothed importance sampling leave-one-out cross-validation. The exploratory analysis reveals a reproducible substructure: the Random Forest achieves $>95\%$ LOOCV accuracy, with $L_X$, $\dot{P}$, and $kT$ emerging as the dominant predictors. However, Bayesian model comparison shows no statistically significant preference for a two-population model. Instead, a few low spin-down sources receive intermediate posterior membership probabilities, suggesting transitional or outlying behavior rather than membership in a distinct class. Overall, current data do not require a separate white-dwarf magnetar population and are adequately described by a predominantly neutron-star population.

Matches the published version in JHEAp: The revised version strengthens the statistical framework, validation, and physical interpretation, reinforcing the conclusion that current data do not support a distinct white-dwarf magnetar population