Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud

Abstract

We report on the long-term average spin period, rate of change of spin period\nand X-ray luminosity during outbursts for 42 Be X-ray binary systems in the\nSmall Magellanic Cloud. We also collect and calculate parameters of each system\nand use these data to determine that all systems contain a neutron star which\nis accreting via a disc, rather than a wind, and that if these neutron stars\nare near spin equilibrium, then over half of them, including all with spin\nperiods over about 100 s, have magnetic fields over the quantum critical level\nof 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then\ntheir magnetic fields are inferred to be much lower, of the order of 10^6-10^10\nG, comparable to the fields of neutron stars in low-mass X-ray binaries. Both\nresults are unexpected and have implications for the rate of magnetic field\ndecay and the isolated neutron star population.\n