Low-frequency QPO from the 11 Hz accreting pulsar in Terzan 5: not frame dragging

TL;DR

This study analyzes RXTE observations of the 11 Hz accreting pulsar in Terzan 5, identifying high-luminosity QPOs that challenge the Lense-Thirring precession explanation, suggesting alternative mechanisms.

Contribution

It provides evidence that low-frequency QPOs in neutron stars can occur without frame dragging, questioning the Lense-Thirring precession model.

Findings

QPOs identified as horizontal branch oscillations (HBO) in the 35-50 Hz range

QPOs occur in a slowly spinning neutron star, inconsistent with frame dragging

Challenges the universality of the Lense-Thirring precession model

Abstract

We report on 6 RXTE observations taken during the 2010 outburst of the 11 Hz accreting pulsar IGR J17480-2446 located in the globular cluster Terzan 5. During these observations we find power spectra which resemble those seen in Z-type high-luminosity neutron star low-mass X-ray binaries, with a quasi-periodic oscillation (QPO) in the 35-50 Hz range simultaneous with a kHz QPO and broad band noise. Using well known frequency-frequency correlations, we identify the 35-50 Hz QPOs as the horizontal branch oscillations (HBO), which were previously suggested to be due to Lense-Thirring precession. As IGR J17480-2446 spins more than an order of magnitude more slowly than any of the other neutron stars where these QPOs were found, this QPO can not be explained by frame dragging. By extension, this casts doubt on the Lense-Thirring precession model for other low-frequency QPOs in neutron-star and perhaps even black-hole systems.