Orbital motion and quasi-quantized disk around rotating neutron stars

Abstract

In accreting neutron star (NS) low-mass X-ray binary (LMXB) systems, NS accretes material from its low-mass companion via a Keplerian disk. In a viscous accretion disk, inflows orbit the NS and spiral in due to dissipative processes, such as the viscous process and collisions of elements. The dynamics of accretion flows in the inner region of an accretion disk is significantly affected by the rotation of NS. The rotation makes NS, thus the space-time metric, deviate from the originally spherical symmetry, and leads to gravitational quadrupole, on one hand. On the other hand, a rotating NS drags the local inertial frame in its vicinity, which is known as the rotational framedragging effect. In this paper, we investigate the orbital motion of accretion flows of accreting NS/LMXBs and demonstrate that the rotational effects of NS result in a band of quasi-quantized structure in the inner region of the accretion disk, which is different, in nature, from the scenario in the strong gravity of black hole arising from the resonance for frequencies related to epicyclic and orbital motions. We also demonstrate that such a disk structure may account for frequencies seen in X-ray variability, such as quasi-periodic oscillations, and can be a potential promising tool of investigation for photon polarization.

A Brief Review for Quasi-Periodic Oscillations in Neutron-Star Low-Mass X-Ray Binaries

“…The relativistic resonance model [102] based on a purely mathematical scheme, offers exact description of the intrinsic oscillatory properties of QPOs, without developments of detailed physical interpretations. Considering the relatively weak gravity of NSs but strong magnetic field and fast rotation, we investigate the rotational effects of NSs in the framework of general relativity, which produce some closure circular orbits in the inner region of accretion disk [100]. We expect that the radial oscillation of plasma, with different fixed frequencies on closure orbits, e.g.…”

“…open circular orbits at each radius, and spiral in towards the NS. If and only if the gravitational Larmor frequency and angular velocity of accreted plasma satisfy[100] ,…”

Physical Environment of Accreting Neutron Stars

“…The former deviates the orbital motion from a circular orbit, and the latter leads to some vertical oscillation. If and only if the gravitational Larmor frequency and angular velocity of accreted plasma satisfy [97] 𝑙𝜔 (𝑟) = 𝑛𝜔 𝐿 ,…”