Disks Surviving the Radiation Pressure of Radio Pulsars

The radiation pressure of a radio pulsar does not necessarily disrupt a surrounding disk. The position of the inner radius of a thin disk around a neutron star, determined by the balance of stresses, can be estimated by comparing the electromagnetic energy density generated by the neutron star as a rotating magnetic dipole in vacuum with the kinetic energy density of the disk. Inside the light cylinder, the near zone electromagnetic field is essentially the dipole magnetic field, and the inner radius is the conventional Alfvén radius. Far outside the light cylinder, in the radiation zone, |E| = |B| and the electromagnetic energy density is 〈S〉/c ∝ 1/r2 where S is the Poynting vector. Shvartsman (1970) argued that a stable equilibrium can not be found in the radiative zone because the electromagnetic energy density dominates over the kinetic energy density, with the relative strength of the electromagnetic stresses increasing with radius. In order to check whether this is true also near the light cylinder, we employ global electromagnetic field solutions for rotating oblique magnetic dipoles (Deutsch 1955). Near the light cylinder the electromagnetic energy density increases steeply enough with decreasing r to balance the kinetic energy density at a stable equilibrium. The transition from the near zone to the radiation zone is broad. The radiation pressure of the pulsar can not disrupt the disk for values of the inner radius up to about twice the light cylinder radius if the rotation axis and the magnetic axis are orthogonal. This allowed range beyond the light cylinder extends much further for small inclination angles. The mass flow rate in quiescent phases of accretion driven millisecond pulsars can occasionally drop to values low enough that the inner radius of the disk goes beyond the light cylinder. The possibilities considered here may be relevant for the evolution of spun-up X-ray binaries into millisecond pulsars, for some transients, and for the evolution of young neutron stars if there is a fallback disk surrounding the neutron star. Subject headings: accretion disks— stars: individual SAX J1808.4–3658, Aquila X–1—stars: neutron—Xrays:binaries