Study on the magnetic field of the ultraluminous X-ray pulsar RX J0209.6-7427

RX J0209.6-7427 is an ultraluminous X-ray pulsar (ULXP) having spin period of about 9.3 s. To date, no cyclotron resonance scattering features have been detected in this source, which can enable direct measurement of the magnetic field of the pulsar. We estimate the surface magnetic field of the neutron star in this source using different models and find that the inferred magnetic field lies in the range of $2.4-4 \times 10^{13}$G. We study the magnetic field and spin period evolution of the source using existing models and find that the magnetic field will decay to about $\sim 10^{9}$G and the source will become a millisecond pulsar at the end of the accretion phase of the accreting binary. Comparison between the magnetic field and the spin period of other ULXPs with those of magnetars suggests that some ULXPs may be accreting magnetars. Studying the magnetic and spin period evolution of ULXPs may be helpful for understanding magnetar evolution and the millisecond pulsar formation.