The spin-down mechanism of the X-ray pulsar 4U 2206+54

Observations of the X-ray pulsar 4U 2206+54 obtrained over 15 years show that its period, which is now 5555 +/- 9 s, is increasing dramatically. This behavior is difficult to explain using traditional scenarios for the spin evolution of compact stars. The observed spin-down rate of the neutron star in 4U 2206+54 is in good agreement with the value expected in a magnetic-accretion scenario, taking into account that, under certain conditions, the magnetic field of the accretion stream can affect the geometry and type of flow. The neutron star in this case accretes material from a dense gaseous slab with small angular momentum, which is kept in equilibrium by the magnetic field of the flow itself. A magnetic-accretion scenario can be realized in 4U 2206+54 if the magnetic-field strength at the surface of the optical counterpart to the neutron star is higher than 70 G. The magnetic field at the surface of the neutron star is 4 x 10(12) G in this scenario, in agreement with estimates based on an analysis of X-ray spectra of the pulsar.