MHD simulations of oscillating cusp-filling tori around neutron stars – missing upper kHz QPO

Abstract

We performed axisymmetric, grid-based, ideal magnetohydrodynamic (MHD) simulations of oscillating cusp-filling tori orbiting a non-rotating neutron star. A pseudo-Newtonian potential was used to construct the constant angular momentum tori in equilibrium. The inner edge of the torus is terminated by a "cusp" in the effective potential. The initial motion of the model tori was perturbed with uniform sub-sonic vertical and diagonal velocity fields. As the configuration evolved in time, we measured the mass accretion rate on the neutron star surface and obtained the power spectrum. The prominent mode of oscillation in the cusp torus is the radial epicyclic mode. It would appear that vertical oscillations are suppressed by the presence of the cusp. From our analysis it follows that the mass accretion rate carries a modulation imprint of the oscillating torus, and hence so does the boundary layer luminosity.

A possible explanation of the parallel tracks in kilohertz quasi-periodic oscillations from low-mass-X-ray binaries

“…A three-dimensional MHD simulation of an accretion process at a quasi-equilibrium state was performed when the gravitational, centrifugal, and pressure gradient force were considered by Kulkarni & Romanova (2013). Parthasarathy et al (2017) also performed an idealised MHD simulation of oscillating cusp-filling tori orbiting a non-rotating axisymmetric NS and they confirmed the modulation mechanism of the neutron star boundary layer luminosity by disc oscillations.…”

“…The MHD waves generated at the magnetosphere radius modulate the luminosity and lead to the kHz QPOs (e.g. Parthasarathy et al, 2017).…”

“…A three-dimensional MHD simulation of an accretion process at a quasi-equilibrium state was performed when the gravitational, centrifugal, and pressure gradient force were considered by Kulkarni & Romanova (2013). Parthasarathy et al (2017) parameters. The explanation of the parallel tracks, in which the slowly varying effective magnetic fields lead to the parallel tracks in a source, is presented.…”

Kilohertz QPOs in low-mass X-ray binaries as oscillation modes of tori around neutron stars – I

“…In this way they were able to match the ν l − νu relation for the kHz QPOs considerably better than previous models, such as the relativistic-precession model (Stella & Vietri 1999), constraining the mass of the neutron star in 4U 1636-53 to be M = 1.69 M . More recently, Parthasarathy et al (2017) performed axisymmetric ideal-MHD Newtonian simulations of oscillating cusp-filling tori orbiting a nonrotating neutron star. They followed the response of the mass-accretion rate to the oscillations of the torus and related this to the boundary layer formed at the surface of the neutron star, finding that the most prominent mode of oscillation in the mass-accretion rate is the radial epicyclic mode, which they associated with the lower kHz QPO; they could not detect, however, the upper kHz QPO.…”

The possible origin of high frequency quasi-periodic oscillations in low mass X-ray binaries

“…They believed that such high coherency is not possible to achieve from kinematic effects in orbital motion. However, in the axisymmetric, ideal magnetohydrodynamic simulation of Parthasarathy et al (2017) for an oscillating cusp-filling tori orbiting a non-rotating neutron star, they considered different eigenmodes lead to the difference of the quality factor of the twin kHz QPOs. In our model, two different modes correspond to the twin HFQPOs and they may also lead to the difference of the quality factor as Parthasarathy et al (2017) obtained, which is an important field to be explored next in the future.…”