Stationary accelerators around Keplerian disks of aligned magnetized collapsed objects: Pair production and gamma-ray emission

Abstract

The magnetosphere is described for a system consisting of a spinning collapsed star connected by an aligned magnetic dipole field (B) to a surrounding idealized coaxial, nonviscous cool Keplerian disk. When the inner part of the conducting disk rotates more rapidly than the star, the magnetosphere is charge-separated. A part of the magnetosphere will corotate either with the star or with that part of the disk to which it is linked by the magnetic field. Separating such differently rotating regions are "gaps" empty of plasma. The magnetic field, including that part which penetrates through the disk, remains everywhere time-independent. Except in the gap and in the disk, the electric field component is E·B ≅ 0. The dynamo electromotive force from the (differently) rotating star and disk connected by B is balanced by the potential drop along B in the gap. The model gap potential drop is about 10 16 V for disk and star parameters of a low-mass X-ray binary (LMXB) and about 10 15 V for those of typical X-ray pulsar binaries. If very hot plasma from the star or the disk does not quench the accelerator, accretion-powered X-rays which traverse the gap limit its accelerating electric field by the e ± pair-production processes which they support. In the resulting steady state a constant current flows through the accelerator which gives a twist to, but does not continually stretch, the magnetic field through the disk. The steady state gap potential drop can accelerate proton (ions) toward the disk with enough energy so that interactions in the disk can give TeV y-rays. For LMXB parameters the model gives strong e ± production and hard X-ray and MeV γ-ray emission with total power near that of accretion-powered X-rays. With X-ray pulsar binary parameters the model total γ-ray power is typically about 10 -2 times the full accretion X-ray power. Possible application to disks around magnetic white dwarfs is considered.