Magnetic field decay and period evolution of the source RX J0720.4-3125

We studied possible evolution of the rotational period and the magnetic field of the X-ray source RX J0720.4-3125 assuming this source to be an isolated neutron star accreting from the interstellar medium. Magnetic field of the source is estimated to be $10^6 - 10^9$ Gs (most probably $\approx 2\cdot 10^8$ Gs), and it is difficult to explain the observable rotational period 8.38 s without invoking hypothesis of the magnetic field decay. We used the model of ohmic decay of the crustal magnetic field. The estimates of the accretion rate ($10^{-14} - 10^{-16} M_\odot/yr$), velocity of the source relative to the interstellar medium ($10 - 50 $ km/s), the neutron star age ($2\cdot 10^9 - 10^{10}$ yrs) are obtained.