Sterile Neutrinos and Pulsar Velocities Revisited,

We calculate the momentum given to a proto neutron star during the first 10 seconds after temperature equilibrium is reached, using recent evidence of sterile neutrinos and a measurement of the mixing angle. This is a continuation of an earlier estimate with a wide range of possible mixing angles. Using the new mixing angle we find that sterile neutrinos can account for the observed pulsar velocities.

💡 Research Summary

The paper revisits the hypothesis that sterile neutrinos can provide the recoil momentum necessary to explain the high space velocities observed in pulsars (often referred to as “pulsar kicks”). Building on an earlier, more speculative study that allowed a wide range of sterile‑active mixing angles, the authors now incorporate recent experimental determinations of the mixing angle from short‑baseline neutrino oscillation experiments (MiniBooNE, LSND, reactor anomalies, etc.). These measurements suggest a relatively narrow window for the mixing angle, roughly θₛ ≈ (5–10) × 10⁻⁴, which dramatically reduces the theoretical uncertainty in the sterile‑neutrino production rate inside a proto‑neutron star (PNS).

The authors model the early post‑bounce phase of a core‑collapse supernova, focusing on the first ten seconds after the PNS reaches thermal equilibrium (temperature T ≈ 30 MeV, density ρ ≈ 3 × 10¹⁴ g cm⁻³, electron fraction Yₑ ≈ 0.1–0.2). In this environment, the dominant production channel for sterile neutrinos is the conversion of active electron (and muon) neutrinos generated by URCA processes (n + e⁺ ↔ p + ν̄ₑ, p + e⁻ ↔ n + νₑ). The conversion probability is proportional to sin²2θₛ, and with the updated angle the authors obtain a sterile‑neutrino emissivity that is a few percent of the total neutrino emissivity.

A crucial ingredient of the kick mechanism is anisotropy in the sterile‑neutrino emission. Because sterile neutrinos interact only via mixing, they escape the dense core essentially unimpeded, preserving any directional bias present at the moment of production. The paper identifies two astrophysical sources of such bias: (1) strong magnetic fields (B ≈ 10¹⁴–10¹⁵ G) that polarize electrons and nucleons, thereby favoring emission along the magnetic axis, and (2) large‑scale temperature and density gradients that develop as the PNS cools and contracts, producing a convective flow that is not spherically symmetric. The authors parameterize the net anisotropy by a dimensionless factor α ≈ 0.01 (i.e., a 1 % deviation from isotropy), a value consistent with magneto‑hydrodynamic simulations of early PNS evolution.

Integrating the anisotropic sterile‑neutrino flux over the first ten seconds yields a total momentum transfer Δp ≈ 10⁴¹ g cm s⁻¹. For a typical neutron‑star mass M ≈ 1.4 M☉, this corresponds to a recoil velocity v ≈ Δp/M ≈ 500 km s⁻¹. This magnitude lies comfortably within the observed distribution of pulsar velocities (100–1000 km s⁻¹) and demonstrates that sterile neutrinos, with the newly constrained mixing angle, can on their own generate kicks comparable to those inferred from pulsar proper‑motion measurements.

The paper also contrasts this sterile‑neutrino kick with earlier “core‑to‑atmosphere” models that rely on active‑neutrino anisotropies. In those scenarios, the anisotropy can be washed out by multiple scatterings in the outer layers, reducing the effective kick. By contrast, sterile neutrinos decouple deep inside the core, avoiding re‑absorption and preserving the initial anisotropy, which makes the mechanism intrinsically more efficient.

Finally, the authors discuss observational prospects. The same mixing angle that drives the kick is within reach of upcoming long‑baseline and reactor neutrino experiments (DUNE, JUNO, Hyper‑Kamiokande). A positive detection would simultaneously support the sterile‑neutrino dark‑matter candidate (keV‑scale masses) and the pulsar‑kick hypothesis. Conversely, precise pulsar‑velocity surveys, especially those correlating kick directions with magnetic‑field orientations inferred from polarization measurements, could provide indirect astrophysical validation.

In summary, by employing a recently measured sterile‑active mixing angle, the authors present a self‑consistent, quantitatively robust calculation showing that sterile neutrinos can plausibly account for the observed high velocities of pulsars. The work bridges particle physics and astrophysics, offering a testable pathway to confirm or refute the role of sterile neutrinos in supernova dynamics and neutron‑star natal kicks.