The Galactic Population of Magnetars: A Simulation-based Inference Study

Population synthesis modeling of the observed dynamical and physical properties of a population is a highly effective method for constraining the underlying birth parameters and evolutionary tracks. In this work, we apply a population synthesis model to the canonical magnetar population to gain insight into the parent population. We utilize simulation-based inference to reproduce the observed magnetar population with a model that takes into account the secular evolution of the force-free magnetosphere and magnetic field decay simultaneously and self-consistently. Our observational constraints are such that no magnetar is detected through their persistent emission when convolving the simulated populations with the XMM-Newton EPIC-pn Galactic plane observations, and that all of the ∼30 known magnetars are discovered through their bursting activity in the last ∼50 yr. Under these constraints, we find, within 95% credible intervals, the birth rate of magnetars to be 1 . 8 − 0.6 + 2.6 kyr⁻¹, leading to having 10 . 7 − 4.4 + 18.8 % of neutron stars born as magnetars. We also find a mean magnetic field at birth (μ_b is in T) log μ b = 10 . 2 − 0.2 + 0.1 , a magnetic field decay slope α d = 1 . 9 − 1.3 + 0.9 , and timescale τ d = 17 . 9 − 14.5 + 24.1 kyr, in broad agreement with previous estimates. We conclude this study by exploring detection prospects: an all-sky survey with XMM-Newton would potentially allow around seven periodic detections of magnetars to be obtained, with approximately 150 magnetars exceeding XMM-Newton’s flux threshold, and the upcoming AXIS experiment should allow these detections to be doubled.