Are the kHz QPO lags in neutron star 4U 1608-52 due to reverberation?

TL;DR

This study investigates whether reverberation explains the kHz QPO lags in neutron star 4U 1608-52, finding that reverberation alone cannot account for the observed lag behaviors, implying different origins for lower and upper kHz QPOs.

Contribution

The paper adapts relativistic ray tracing models from AGN to neutron star binaries and compares predicted and observed lags, revealing limitations of reverberation as the sole explanation.

Findings

Reverberation models predict lag energy spectra increasing above 8 keV.

Observed lags in 4U 1608-52 decrease with energy, inconsistent with reverberation.

Models better match the flatter lag spectrum of upper kHz QPOs.

Abstract

X-ray reverberation lags have recently been discovered in both active galactic nuclei (AGN) and black hole X-ray binaries. A recent study of the neutron star low-mass X-ray binary 4U 1608-52 has also shown significant lags, whose properties hint at a reverberation origin. Here, we adapt general relativistic ray tracing impulse response functions used to model X-ray reverberation in AGN for neutron star low-mass X-ray binaries. Assuming relativistic reflection forms the broad iron line and associated reflection continuum, we use reflection fits to the energy spectrum along with the impulse response functions to calculate the expected lags as a function of energy over the range of observed kHz QPO frequencies in 4U 1608-52. We find that the lag energy spectrum is expected to increase with increasing energy above 8 keV, while the observed lags in 4U 1608-52 show the opposite behavior. This demonstrates that the lags in the lower kHz QPO of 4U 1608-52 are not solely due to reverberation. We do note, however, that the models appear to be more consistent with the much flatter lag energy spectrum observed in the upper kHz QPO of several neutron star low-mass X-ray binaries, suggesting that lower and upper kHz QPOs may have different origins.