How to make a mature accreting magnetar
Several candidates for accreting magnetars have been proposed recently by different authors. Existence of such systems contradicts the standard magnetic field decay scenario where a large magnetic field of a neutron star reaches ≲a few ×1013 G at ages ≳1 Myr. Among other sources, the high-mass X-ray binary 4U 0114+65 seems to have a strong magnetic field around 1014 G. We develop a new Bayesian estimate for the kinematic age and demonstrate that 4U 0114+65 has kinematic age 2.4-5 Myr (95 per cent credential interval) since the formation of the neutron star. We discuss which conditions are necessary to explain the potential existence of magnetars in accreting high-mass binaries with ages about few Myr and larger. Three necessary ingredients are: the Hall attractor to prevent rapid decay of dipolar field, relatively rapid cooling of the crust in order to avoid Ohmic decay due to phonons, and finally, low values of the parameter Q to obtain long Ohmic time-scale due to impurities. If age and magnetic field estimates for proposed accreting magnetars are correct, then these systems set the strongest limit on the crust impurity for a selected sample of neutron stars and provide evidence in favour of the Hall attractor.