The sterile neutrino mechanisms for natal neutron star kicks are reanalyzed. It is shown that the magnetic field strengths needed for obtaining the observable values of kicks were underestimated essentially. Another mechanism with standard neutrinos is discussed where the outgoing neutrino flux in a supernova explosion with a strong toroidal magnetic field generation causes the field redistribution in "upper" and "lower" hemispheres of the supernova envelope. The resulting magnetic field pressure asymmetry causes the pulsar natal kick.
The problem of large proper velocities of pulsars, born in supernova explosions (pulsar kick), has been discussed for more than 40 years. The total list of publications with the observational data is very long. We indicate here only the first papers [1,2] where the problem was put, and the papers where the data were summarized with the samples of 99 pulsars [3] and of 233 pulsars [4]. Average speed for the sample of 233 pulsars [4] was estimated at the level of 400 km/s, with more than 15 % having velocities greater than 1000 km/s. The two fastest pulsars PSRs B2011+38 and B2224+64 have ∼ 1600 km/s.
It is important that a correlation was finally established between the directions of pulsar velocities and of rotation axes. Initially, a conclusion was made in the paper [5], based on an analysis of the set of 29 pulsars, that mechanisms predicting a correlation between the rotation axis and the pulsar velocity were ruled out by the observations. However, in the paper [6] strong observational evidence was presented for a relationship between the direction of a pulsar’s motion and its rotation axis. Analysing a set of 25 pulsars which are younger than the ones taken in Ref. [5], the authors [6] conclude that 10 pulsars show an offset between the velocity vector and the rotation axis, which is either less than 10 0 or more than 80 0 , a fraction that is very unlikely by random chance.
Obviously, the reason for the initial kick is a kind of an asymmetry in a supernova explosion, but the nature of it has not yet disclosed. There were many attempts to explain this asymmetry.
Numerous attempts to describe the effect in the hydrodynamics of a supernova explosion do not explain the large speeds. Three-dimensional simulation of the explosion with the assumption of initial asymmetry in the supernova core before the collapse, which increases during the collapse, leads to the velocity of a pulsar not more than 200 km/s [7]. Multidimensional simulation by H.-T. Janka e.a. [8] where the explosion anisotropies develop chaotically, resulted in a possible pulsar velocity of 10 3 km/s. However, there was no correlation between the direction of pulsar velocity and the rotation axis direction [6] in this approach.
Along with the hydrodynamic approach, there were several different ideas to explain the velocities of pulsars, but all of them operated at speeds of scale of 100 km/s: i) evolution of close binary systems [9]; ii) acceleration of a pulsar within a few months after the explosion due to asymmetric electromagnetic radiation caused by the inclination of the magnetic moment with respect to the axis of rotation and the displacement of the center of the star [10];
iii) asymmetric radiation of neutrinos (antineutrinos) in a collapse via the URCA-processes in a strong magnetic field of the scale of 10 14 -10 15 G in a supernova core [11][12][13].
The neutrino mechanism looks the most interesting. It is known that neutrinos carry away 99 % of the total supernova energy E ∼ 3 × 10 53 erg. When the asymmetry is of ∼ 3 %, neutrinos carry the momentum of ∼ 0.03 E/c. The compact explosion remnant, a neutron star with a mass ∼ 1.4M ⊙ , gets the same momentum. In this case, its velocity can be easily estimated as ∼ 1000 km/sec.
However, neutrinos produced in the electroweak processes have small mean free paths in matter of the central part of a supernova and may not cause high-velocity pulsars [14][15][16].
A lively discussion was generated by the idea [17], under which the neutrino flux asymmetry from a protoneutron star arose due to neutrino oscillations in matter and intensive magnetic field. The neutrinosphere for ν τ lies inside the neutrinosphere for ν e , and the resonant transition ν e → ν τ is possible under certain conditions in the region between the neutrinospheres, where ν e are entangled in the medium while ν τ are “free” to depart. Hence the surface of the resonant transition becomes an effective neutrinosphere for ν τ . In the presence of a magnetic field, this sphere is deformed along the field. Due to the temperature dependence on the radius, the anisotropy of the energy flux carried away by neutrinos, arises. This should cause the kick of the nascent neutron star.
The idea of the pulsar kick due to deformed neutrinosphere [17] raised, however, serious criticism [18]: after the neutrinosphere deformation, the surfaces of the constant temperature would be deformed also, because just neutrinos provided a thermal equilibrium. And the main problem of the model became clear soon: the existence of neutrinos with the mass ∼ 100 eV was needed. Established restriction on the neutrino mass, m ν < 2 eV, “closed” the model.
There were also attempts to explain large space velocities of young pulsars with using of some possible non-standard properties of neutrinos. For example, a mechanism was proposes by E. Akhmedov e.a. [19], of the resonant spin-flavour precession of neutrinos with a transition magnetic moment in the mag
This content is AI-processed based on open access ArXiv data.
