RX J0720.4-3125 as a possible example of magnetic field decay in neutron stars

We consider the possible evolution of the rotation period and magnetic field of the X-ray source RX J0720.4-3125, assuming that this source is an isolated neutron star accreting from the interstellar medium. The magnetic field of the source is estimated to be 10⁶–10⁹ G (the most probable value is about 2·10⁸ G), and it is difficult to explain the observed rotational period 8.38 s without invoking the hypothesis of magnetic field decay. For calculations we used the model of ohmic dissipation of the field in the core of the neutron star. Estimates for the accretion rate (10⁻¹⁴–10⁻¹⁶ M_⊙/yr), velocity of the source through the interstellar medium (10–50 km/s), and neutron star age (2·10⁹–10¹⁰ yrs) are obtained. Lomonosov State University, Moscow, Russia; loffe Physical and Engineering Institute, St. Petersburg, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 1, pp. 28–35, January, 1998.