Author(s): Tate Thomas
Mentor(s): Alexander M. Panin
Institution UVU
Observations have shown that some neutron stars exhibit velocities around 200-500 km/s on average, reaching speeds over 1000 km/s in some instances. This cannot be attributed to the orbital velocity of the neutron star around our Milky Way galaxy, as the average orbital velocity is around 250 km/s. The origin of this phenomenon, known as a pulsar kick, is not fully understood and remains a subject of debate among astrophysicists. Due to a star’s rotation and its strong (and often asymmetric) magnetic field, it is unrealistic to expect a spherically symmetric collapse of the star’s core plasma. This asymmetric collapse may result in the formation of dense neutron matter slightly offset from the center of the forming neutron star, of which will be a source of subsequent high-energy neutrino radiation. As a consequence of this offset, we would expect an imbalance of neutrino radiation reaching opposite sides of the neutron star, creating a net momentum of neutrinos radiated into space and propelling the neutron star in the opposite direction. Moreover, in a collapsing, spinning progenitor star, the neutrino offset will most likely be aligned with the angular momentum of the star’s core. Indeed, recent observations show that there is a strong correlation of direction of kick velocity with the spin of observed neutron stars. We investigated the diffusion of hot (~10-100 MeV) neutrinos via dense neutron matter from an offset source inside of the forming neutron star, both analytically and numerically. Our results show that the diffusing neutrinos do, in fact, escape into space in a highly anisotropic manner. Furthermore, we derived a formula for the pulsar kick velocity based on the neutrino formation offset, which predicts kickback velocities due to small asymmetries (5-10% offset from the neutron star’s radius) consistent with observational data. Our findings suggests that asymmetric neutrino emission can account for significant recoil velocities, providing a plausible explanation for pulsar kicks.