Speaker
Description
The simultaneous presence of kilohertz quasi-periodic oscillations (kHz QPOs) and relativistic Fe K emission lines in neutron star Low-Mass X-ray Binaries offers a unique opportunity to probe the innermost accretion flow. The frequency of a kHz QPO is thought to trace the orbital motion of matter in the inner accretion disk, while the Fe K line profile, shaped by relativistic broadening, provides a geometric probe of the same region. If both signals originate from the same radius, timing and spectroscopy provide independent diagnostics that can be directly translated into a neutron star mass measurement through the definition of the gravitational radius.
However, current observatories lack the effective area required to measure the Fe line and track kHz QPOs on comparable timescales. Long integrations are needed to model the relativistic line with sufficient precision, during which the QPO frequency may drift and the continuum and illumination conditions may evolve, washing out any intrinsic correlation.
NewAthena will overcome these limitations by combining a large collecting area with high energy resolution. This will enable simultaneous timing and spectroscopic constraints on kilosecond timescales and allow a direct comparison between inner disk radii inferred from Fe K line modeling and from kHz QPO frequencies across different spectral states.
We present dedicated NewAthena simulations of bright neutron star LMXBs in which both relativistic disk reflection and kHz QPOs have been robustly detected, such as 4U 1636–53, 4U 1608–52, and 4U 1820–30. Our results demonstrate that the inner disk radius can be constrained in a few kiloseconds, avoiding the long integrations currently required and enabling a self-consistent test of whether dynamical and spectroscopic radii track each other in the inner accretion flow.
This work has been accepted for publication as part of the JHEAP Special Issue dedicated to the NewAthena science case.
