We report on the long-term average spin period, rate of change of spin period
and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the
Small Magellanic Cloud. We also collect and calculate parameters of each system
and use these data to determine that all systems contain a neutron star which
is accreting via a disc, rather than a wind, and that if these neutron stars
are near spin equilibrium, then over half of them, including all with spin
periods over about 100 s, have magnetic fields over the quantum critical level
of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then
their magnetic fields are inferred to be much lower, of the order of 10^6-10^10
G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both
results are unexpected and have implications for the rate of magnetic field
decay and the isolated neutron star population.Comment: 22 pages, 50 figures; to appear in MNRA
