Exploring the accretion-induced evolution of the spin period and magnetic field strength of Be/X-ray Pulsars

Abstract

Based on the detected spin periods ( $P$ ) and inferred magnetic field strengths ( $B$ ) by the cyclotron resonance scattering features, we analyze the $B-P$ properties of Be/X-ray Pulsars (BeXPs). We find that the $P$ distribution of BeXPs exhibits a bimodal feature separated at $P\sim 40$ s, where the average spin period of the BeXPs with $P>40$ s ( $\langle P\rangle \sim 267$ s) is larger than that of the sources with $P<40$ s ( $\langle P\rangle \sim 10$ s) by about one magnitude of order. Meanwhile, the average magnetic field strength of the long period BeXPs ( $\langle B\rangle \sim 4.9\times 10^{12}$ G) is higher than that of the short period sources ( $\langle B\rangle \sim 2.7\times 10^{12}$ G) by a factor of $\sim 2$ . We try to explain these phenomena by the accretion-induced evolution process, and find that for the neutron star (NS) with the initial magnetic field strength of $B_{0}\sim 10^{12.2}-10^{13}$ G, when it accretes about $\Delta M\sim 10^{-6.5}\,\mathrm{M_{\odot }}$ companion matter, its spin period can shorten from $P_{0}\sim 1000$ s to $P\sim 260$ s, while its magnetic field strength decays little. Furthermore, when the NS accretes about $\Delta M\sim 10^{-5.5}\,\mathrm{M_{\odot }}$ matter, its spin period can shorten to $P\sim 10$ s, while its magnetic field strength decays by half. Finally, we also notice that as the continuing of the accretion process in Be/X-ray binary, when its NS accretes about $\sim 10^{-3}\,\mathrm{M_{\odot }}$ mater, it has the possibility to evolve to the double neutron star.