Speaker
Description
Quasi-periodic oscillations (QPOs) are coherent X-ray timing features frequently observed in black hole X-ray binaries (BH-XRBs), originating from the innermost region of the accretion disc-corona system where general relativistic effects are expected to be in play. Among the proposed QPO models, the Lense-Thirring (LT) precession of the inner hot flow predicts a systematic modulation of the fluorescent iron line energy over the QPO phase (Ingram et al., 2012, MNRAS, 427, 934). In this work, we test this prediction using NuSTAR observations of seven transient BH-XRBs exhibiting coherent QPOs and prominent iron line features in the hard/intermediate state. Using advanced X-ray timing analysis techniques, we construct QPO phase-resolved spectra over multiple phase bins. Detailed phase-resolved spectral modeling with thermal Comptonization and relativistic reflection models reveals evidence of systematic 'rocking' of the iron line centroid energy in the red- and blue-shifted regimes over the QPO phase, with significance varying within 1-3.7 \sigma. These results provide strong support for frame-dragging effects in the vicinity of the black holes. We also find systematic correlations/anti-correlations of spectral parameters, such as photon index, electron temperature, disc ionization, and optical depth, with the QPO waveform, regulated by the observed QPO frequency. These findings broadly favor the origin of QPOs through the precession/modulation of the X-ray corona, where illumination of different azimuths of the accretion disc by the inner hot coronal emission produces the observed variations in the iron line centroid energy and associated spectral parameters as seen by a distant observer. Finally, we discuss the implications of our results and the prospects for extending such studies with current state-of-the-art X-ray missions such as XRISM.