Can accretion onto isolated neutron stars produce γ-ray bursts?

Abstract

THE isotropy and flat count spectrum of γ-ray bursts revealed by the BATSE detector on the Compton Gamma-Ray Observatory¹ have led to suggestions that the burst sources are an extended galactic halo of high-velocity neutron stars^2,3We show here that if slow accretion onto these neutron stars from the interstellar medium is to be the origin of γ-ray bursts, the accretion physics is very different from what applies for local, low-velocity neutron stars^4,5. For halo neutron stars with high magnetic fields and velocities (v > 190 km s^–1), electromagnetic dipole radiation pressure prevents accretion unless the period is longer than tens of seconds; the centrifugal barrier will then prevent accretion until the period reaches several thousand seconds. For periods as long as this, accretion may proceed through Kelvin–Helmholtz instability at the magnetopause boundary. At interstellar densities and neutron-star magnetic fields of ∼10¹² G, the accretion rate by this process can be much larger than the Bondi–Hoyle (hydrodynamic) accretion rate, but is still well below what is needed for slow-accretion burst models. We conclude that slow accretion onto high-velocity neutron stars in the halo cannot be the origin ofγ-ray bursts.