Magnetic Field Decay and Period Evolution of the Source Rx J0720.4-3125

We studied possible evolution of rotational period and magnetic field of the X-ray source RX J0720.4-3125 assuming this source to be an isolated neutron star accreting from interstellar medium. Magnetic field of the source is estimated to be 10 6 − 10 9 Gs (most probably ≈ 2 · 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 crustal magnetic field. The estimates of the accretion rate (10 −14 − 10 −16 M ⊙ /yr), velocity of the source relative to the interstellar medium (10−50 km/s), the neutron star age (2·10 9 −10 10 yrs) are obtained. (remark added in proof: As we found out in July 1997 when this paper was already submitted, common results were recieved independently by John C.L. Wang // Astrophys.