Very low-frequency oscillations from the 11 Hz pulsar in Terzan 5: frame-dragging back on the table

TL;DR

This study re-analyzed X-ray data from the 11Hz pulsar in Terzan 5, discovering very low-frequency QPOs consistent with frame-dragging effects, challenging previous classifications of higher-frequency oscillations.

Contribution

It presents the first detection of very low-frequency QPOs in this pulsar, supporting the relativistic precession model and revising the understanding of QPO origins in neutron star systems.

Findings

Detected seven new QPOs below 0.3 Hz.

QPOs are consistent with frame-dragging predictions.

Reinterpreted higher-frequency QPOs as possibly related to different phenomena.

Abstract

We present a re-analysis of 47 Rossi X-ray Timing Explorer observations of the 11Hz accreting pulsar IGR J17480-2446 in Terzan 5 during its 2010 outburst. We studied the fast-time variability properties of the source and searched for quasi-periodic oscillations (QPOs) in a large frequency range. General Relativity predicts that frame-dragging occurs in the vicinity of a spinning compact object and induces the precession of matter orbiting said object. The relativistic precession model predicts that this frame-dragging can be observed as QPOs with a characteristic frequency in the light curves of accreting compact objects. Such QPOs have historically been classified as horizontal branch oscillations in neutron star systems, and for a neutron star spinning at 11 Hz these oscillations are expected at frequencies below 1 Hz. However, previous studies of IGR J17480-2446 have classified QPOs at 35-50 Hz as horizontal branch oscillations, thus casting doubts on the frame-dragging nature of such QPOs. Here we report the detection of seven very low-frequency QPOs, previously undetected, with centroid frequencies below 0.3 Hz, and which can be ascribed to frame-dragging. We also discuss the possible nature of the QPOs detected at 35-50 Hz in this alternative scenario.