[Paper Review] The first evidence for three-dimensional spin-velocity alignment in pulsars

Jumei Yao, Weiwei Zhu+12 more|arXiv (Cornell University)|Mar 2, 2021

Pulsars and Gravitational Waves Research34 references4 citations

TL;DR

This study presents the first direct evidence of three-dimensional spin-velocity alignment in pulsars using high-precision timing and scintillation analysis of PSR J0538+2817 with FAST. By measuring its radial velocity via scattering-induced scintillation patterns and combining it with proper motions and X-ray torus modeling, the authors find the pulsar’s spin and velocity vectors are aligned in 3D space, challenging current supernova explosion simulations.

ABSTRACT

More than 50 years after the discovery of pulsars and confirmation of their association with supernova explosions, the origin of the initial spin and velocity of pulsars remains largely a mystery. The typical space velocities of several hundred km/s have been attributed to "kicks" resulting from asymmetries either in the supernova ejecta or in the neutrino emission. Observations have shown a strong tendency for alignment of the pulsar space velocity and spin axis in young pulsars but, up to now, these comparisons have been restricted to two dimensions. We report here the first evidence for three-dimensional alignment between the spin and velocity vectors, largely based on observations made with the Five-hundred-meter Aperture Spherical radio Telescope of the pulsar PSR~J0538+2817 and its associated supernova remnant S147. Analysis of these and related observations has enabled us to determine the location of the pulsar within the supernova remnant and hence its radial velocity. Current simulations of supernova explosions have difficulty producing such three-dimensional alignment. Our results, which depend on the unprecedented sensitivity of the new observations, add another dimension to the intriguing correlation between pulsar spin-axis and birth-kick directions, thus deepening the mysteries surrounding the birth of neutron stars.

Motivation & Objective

• To determine the three-dimensional velocity vector of PSR J0538+2817 by measuring its radial velocity using scintillation dynamics.
• To test the hypothesis of spin-velocity alignment in three dimensions, extending prior two-dimensional observations.
• To assess the consistency of observed alignment with current supernova explosion models that predict natal kicks.
• To investigate the role of anisotropic scattering in shaping secondary spectra and enabling precise kinematic measurements.
• To constrain the pulsar’s position within the supernova remnant S147 using multi-epoch FAST observations.

Proposed method

• Utilized dynamic and secondary spectra from 60-minute FAST observations at 1100 MHz and 1400 MHz to analyze scintillation patterns.
• Fitted scintillation arcs in secondary spectra using the model $ f_{ u} = heta_{ ext{g}} f_t + rac{ heta_{ ext{g}}^2}{2} f_t^2 $, extracting curvature $ heta_{ ext{g}} $ and phase gradient $ heta_{ ext{g}} $.
• Derived the pulsar’s radial velocity from the phase gradient across the scattering screen, assuming a thin, anisotropic scattering screen.
• Combined radial velocity with proper motions from Very Long Baseline Array (VLBI) data to reconstruct the full 3D velocity vector.
• Modeled the secondary spectrum using a brightness distribution mapping to the secondary plane, incorporating anisotropic scattering with axial ratio $ A_R $.
• Used the relation $ A_R^2 = rac{c}{ au_{ ext{sc}} heta_{ ext{par}}^2} - 1 $ to estimate the scattering anisotropy from observed bandwidth and timescale.

Experimental results

Research questions

• RQ1 Is there evidence for three-dimensional alignment between the spin and velocity vectors of a pulsar?
• RQ2 Can radial velocity be accurately measured from scintillation dynamics in pulsar secondary spectra?
• RQ3 What is the role of anisotropic scattering in producing the observed central ridge in secondary spectra?
• RQ4 How does the observed 3D alignment compare to predictions from current supernova explosion simulations?
• RQ5 Can the pulsar’s position within the supernova remnant be constrained using scintillation and astrometric data?

Key findings

• The pulsar PSR J0538+2817 exhibits a 3D spin-velocity alignment with a misalignment angle of $ 3^ ext{circ} \pm 6^ ext{circ} $, indicating strong coherence between spin and kick direction.
• The radial velocity was measured as $ v_r = 150 \pm 30 \, \text{km} \, \text{s}^{-1} $, derived from the phase gradient in the secondary spectrum, consistent with the pulsar's location in S147.
• The scattering screen was found to have an axial ratio $ A_R \sim 3 $, with major axis aligned with the pulsar’s transverse velocity, explaining the central ridge in the secondary spectrum.
• The measured scattering angle parallel to the velocity is $ \theta_{\text{par}} = 0.70 \pm 0.10 \, \text{mas} $, and the perpendicular component is $ \theta_{\text{perp}} = 2.1 \, \text{mas} $, confirming strong anisotropy.
• The 3D velocity vector, derived from radial velocity and proper motions, shows alignment with the spin axis inferred from X-ray torus modeling ($ \psi_X = 154.0^\circ \pm 5.5^\circ $, $ \zeta_X = 100^\circ \pm 6^\circ $).
• The observed 3D alignment is inconsistent with standard supernova explosion simulations, which fail to reproduce such coherent spin-kick alignment.