Be born slow or die fast: spin evolution of neutron stars with alignment or counteralignment

Abstract

We revisit the case of magneto-rotational evolution of neutron stars (NSs) with accounting for changes of the angle χbetween spin and magnetic axes. This element of the evolution of NSs is very important for age estimates and population modeling, but usually it is neglected. In the framework of two models of energy losses we demonstrate that unless NSs are born with the inclination angle χ_0 very close (within 1 degree) to the position of maximal losses, pulsars with short initial periods quickly reach regions of small energy losses (aligned rotators in the case of magneto-dipole losses and orthogonal rotators -- in the case of current losses) without significant spin-down. This means that either most of known NSs should be born with relatively long periods close to presently observed values, or the initial inclination angles should be large (close to 90) for the magnetodipole model and very small (close to 0) for the current losses model, or that both models cannot be applied for the whole evolutionary track of a typical NS. In particular, magnetar candidates should be born with periods close to the observed ones within the scope of both models of energy losses. We discuss how these considerations can influence population synthesis models and age estimates for different types of isolated NSs. Focusing on the model of current losses we illustrate our conclusion with evolutionary track reconstructions on the P--\sin χplane. We conclude, that most probably both existing models of energy losses -- magneto-dipole and longitudinal current -- require serious modifications, on the other hand, estimates obtained under the standard assumption of \sin χ=1=const does not have solid theoretical ground and should be taken with care.