Dependences of radio pulsar parameters on the kick velocity

Abstract

According to several studies, analysis of observational data and theoretical modeling favor a bimodal distribution of the natal velocity kick of neutron stars. We analyze this proposal by using available data on radio pulsars. For $\sim200$ normal isolated radio pulsars with well-measured spin and kinematic parameters, we determine if they belong to the low- or high-velocity mode of such a distribution by applying the parametrization proposed by Igoshev (2020). Our results demonstrate that about $23\%$ belong to the low-velocity mode. We then analyze the differences in the properties of the two sets of pulsars belonging to the two modes. For some parameters (characteristic ages and distances), we see a clear difference between the two modes. However, for these quantities, it can be attributed to selection bias. For those parameters that are not subject to strong selection, such as pulse width, we do not observe any difference. Interestingly, we detect a notable difference in the magnetic field distribution between the two modes. Lower-field pulsars ($B\lesssim 10^{12}$~G) are overabundant among objects from the low-velocity mode. Among pulsars with low fields ($\lesssim 10^{11}$~G), we do not identify any objects from the high-velocity mode of the kick distribution. The origin of this discrepancy is not clear, and we discuss several possibilities. Our analysis demonstrates that, most probably, this feature can be explained by selection effects. Thus, we conclude that there is no robust bimodality in physical parameters of radio pulsars that can be related to the proposed bimodality of the kick velocity. This can be considered as an indirect argument against the hypothetical bimodality in the NS kick distribution. (abridged)