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

Abstract

There have been many efforts to explain the dynamical mechanisms behind the phenomenology of quasi-periodic oscillations (QPOs) seen in the X-ray light curves of low-mass X-ray binaries. Up to now, none of the models can successfully explain all the frequencies observed in the power spectral density of the light curves. After performing several general-relativistic hydrodynamic simulations of non-self-gravitating axisymmetric thick tori with constant specific angular momentum oscillating around a neutron star such as the one associated with the low-mass X-ray binary 4U 1636 − 53, we find that the oscillation modes give rise to QPOs similar to those seen in the observational data. In particular, when matching pairs of kilohertz QPOs from the numerical simulations with those observed, certain combinations reproduce well the observations, provided we take a mass for the neutron star that is smaller than what is generally assumed. At the same time, we find that tori with constant specific angular momentum cannot match the entire range of frequencies observed for 4U 1636−53 due to physical constraints set on their size. Finally, we show that our results are consistent with the observed shifts in QPO frequency that could accompany state transitions of the accretion disc.

Non-linear dynamics of hydrodynamic tori as a model of oscillations and bending waves in astrophysical discs

“…High-frequency QPOs sometimes exhibit frequency ratios that have been modelled as the resonances occurring between orbital, vertical and radial oscillations (e.g. Abramowicz & Kluźniak 2001;Rezzolla et al 2003;Fragile et al 2016;de Avellar et al 2018).…”

Constrained transport and adaptive mesh refinement in the Black Hole Accretion Code

“…The main target application of BHAC is the simulation of accretion onto compact objects in arbitrary spacetimes. This has allowed to simulate accretion onto Kerr black holes to build theoretical models of M 87 (Davelaar et al 2019), but also to explore the consequences of alternative theories of gravity (Mizuno et al 2018) or of the presence of a boson star ) at the Galactic Center in the forthcoming horizon-scale VLBI images of Sgr A*, as well as the study of quasi-periodic oscillations in accretion discs around neutron stars (de Avellar et al 2018).…”

Slowly rotating topological neutron stars: universal relations and epicyclic frequencies

“…There are many theories as to the origin of some quasi-periodic oscillations (QPOs) observed in the X-ray light curves of accreting compact objects, but there is no consensus which is the correct one (see [45] for a comprehensive review). The oscillations themselves are on the order of a few hundred Hz to kHz and most models are based either on orbital and epicyclic motion of matter [18,26] or oscillations and instabilities in an accretion disc [27,31] both of which are expected to occur close to the ISCO radius. Nevertheless, almost all QPOs models are related in some way or another to the ISCO radius and the epicyclic frequencies.…”