Constraining the Pulsar 3D Velocity Distribution: The Impact of Spin-Velocity Alignment

TL;DR

This study reconstructs the 3D velocity distribution of pulsars considering spin-velocity alignment, revealing that geometric assumptions significantly influence population inferences and emphasizing the need for larger, more precise datasets.

Contribution

It introduces a hierarchical Bayesian method to incorporate spin-velocity alignment in pulsar velocity distribution modeling, comparing multiple models with current data.

Findings

A Gamma distribution adequately models pulsar velocities with a peak at 237 km/s.

Alignment assumptions lead to systematically lower reconstructed velocities compared to isotropy.

Current data cannot decisively discriminate among velocity models due to limited resolving power.

Abstract

Quantifying the natal kick distribution of pulsars is essential for understanding supernova physics and binary evolution, yet measurements are historically limited by the lack of radial velocity data. Most previous studies rely on transverse velocities under the assumption of spatial isotropy. In this work, we reconstruct the intrinsic three-dimensional (3D) velocity distribution for a curated sample of 18 pulsars by explicitly incorporating the observational constraint of spin-velocity alignment. Using a hierarchical Bayesian framework that accounts for measurement uncertainties, we compare nine candidate velocity distribution models. We find that a Gamma distribution provides an adequate description of the inferred 3D velocities; however, the modest Bayes factor (1.65 relative to a single Maxwellian) indicates that the current data lack sufficient resolving power to discriminate decisively among the models considered. The Gamma model is characterized by a peak velocity of $237^{+67}_{-84}$ km s$^{-1}$. The reconstructed 3D velocities under alignment are systematically lower than those inferred under isotropy, indicating that projection effects can bias individual kick estimates high, while leaving the overall population scale largely unchanged within uncertainties. A complementary analysis of 465 pulsars with transverse velocity estimates favors a Log-Normal distribution for the full sample, while isolated young pulsars remain consistent with a Gamma-like profile. Our results underscore the importance of geometric assumptions in population inference and highlight the need for larger samples with improved distance and spin-axis measurements to place tighter constraints on natal kick physics.