How to make a mature accreting magnetar

TL;DR

This paper investigates the existence of accreting magnetars, proposing conditions under which they can retain strong magnetic fields over several million years, challenging standard magnetic decay models.

Contribution

It introduces a Bayesian method to estimate the age of accreting magnetars and discusses the necessary physical conditions for their magnetic field longevity.

Findings

4U0114+65 has a kinematic age of 2.4-5 Myr.

Accreting magnetars can exist if Hall attractor and crust cooling conditions are met.

The systems constrain crust impurity levels and support the Hall attractor hypothesis.

Abstract

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 $\lesssim$ few$\times 10^{13}$G at ages $\gtrsim 1$ Myr. Among other sources, the high mass X-ray binary 4U0114+65 seems to have a strong magnetic field around $10^{14}$ G. We develop a new Bayesian estimate for the kinematic age and demonstrate that 4U0114+65 has kinematic age 2.4-5 Myr ($95\%$ 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 Myrs 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.