Luminosities of Disk-Accreting non-magnetic neutron stars

Abstract

Disk accretion onto a neutron star possessing a weak surface magnetic field (B ≤ 108 G) provides interesting X-ray emission scenarios, and is relevant for understanding X-ray bursters and low-mass X-ray binaries (LMXBs). The stan- dard (Newtonian) theory of disk-accretion predicts that the matter spiralling in from infinity loses one-half of its total gravitational energy in the extended disk, and the remainder in a narrow boundary layer girdling the neutron star. The ratio of the boundary layer luminosity to that from the disk (EBL/ED) is, there- fore, unity. On incorporation of general relativity without rotation (Schwarzschild solution), EBL/ED is seen to be as high as 6. We construct rotating sequences of neutron stars for three representative equations of state. We show here that for a neutron star rotating at a limit where centrifugal force balances the inward gravitational force, EBL/ED ∼ 0