From Be X-Ray Binaries to Double Neutron Stars: Exploring the Spin and Orbital Evolution

We explore the evolutionary link between Galactic Be X-ray binaries (BeXBs) and Galactic field double neutron stars (DNSs) based on the properties of the NS spin period—P and binary orbital period—Porb. First, both BeXB and DNS sources show positive correlation trends in P versus Porb relation (P−Porb diagram), which may relate to the influence of the accretion evolution. Secondly, the two types of sources exhibit similar bi-modal P/Porb distributions with the gaps of P∼40 s/Porb∼60 days for BeXBs and P∼50 ms/Porb∼1 day for DNSs, respectively. We propose a possibility that Galactic BeXBs may transfer the bi-modal P/Porb classifications to Galactic field DNSs during the evolution process. Furthermore, based on the bi-modal gaps, we infer the contraction factors of P (by ∼1800) and Porb (by ∼160) valuesin the evolution from Galactic BeXBs to Galactic field DNSs. We find that the scaled P or Porb values of Galactic BeXBs share the similar bi-modal distributions to that of Galactic field DNSs, and these results can offer a reference to trace the classification and evolution between the two types of sources.