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

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.