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

ABSTRACT We present a re-analysis of 47 Rossi X-ray Timing Explorer observations of the 11 Hz 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 (NS) systems, and for a NS 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.