Be X-Ray Binaries as Progenitors of Double Neutron Stars: Evidence of Similar bi-modal Classifications

Abstract We explore the evolutional links between Be X-ray binaries (BeXBs) and double neutron stars (DNSs) by analyzing their physical parameters and classifications. We find that both BeXBs and DNSs show positive correlation trends between the neutron star (NS) spin period– P and binary orbital period– P orb , which may relate to the influence of binary accretion. In addition, BeXBs show a bi-modal classification divided by P ∼ 40 s/ P orb ∼ 60 days, where BeXBs with P < 40 s/ P orb < 60 days exhibit stronger accretion-induced properties, e.g., the lower average NS magnetic field strength, than those with P > 40 s/ P orb > 60 days. Similarly, DNSs exhibit a bi-modal classification divided by P orb ∼ 1 day, where DNSs with P orb < 1 day share the stronger accretion-induced properties, e.g., the higher average accretion rate of the recycled NSs, than those with P orb > 1 day. The multi-parameter analysis with the new aggregated data set improves the knowledge of the bi-modal classifications of BeXBs/DNSs. In addition, we consider the influence of the common envelope phase/supernova kick of the donor star on the evolution from BeXBs to DNSs, and further discuss their compatibility with the observed bi-modal classifications of BeXBs/DNSs. Last, we propose a potential evolutional relation between the bi-modal BeXBs and DNSs.