Speaker
Description
Recent timing studies of black hole X-ray binaries have revealed a narrow drop in the coherence between soft (<2 keV) and hard (>2 keV) bands in the 1–5 Hz range during soft-to-hard transitions in outburst decay, at lower luminosities than the hard-to-soft transitions observed during outburst rise. A sudden jump in the phase lags accompanies this coherence drop at the same frequency. Using a novel approach that models the power and cross spectra with combinations of incoherent Lorentzians, we found compelling evidence that these features are caused by a narrow quasi-periodic oscillation (QPO) with intrinsic lags distinct from the underlying broadband noise.
In-depth studies of this phenomenon across multiple sources—MAXI J1348-630, MAXI J1820+070, Cygnus X-1 (NICER), and Swift J1727.8-1613 (XMM-Newton)—yield consistent results. The coherence drops, phase-lag jumps, and the energy-dependent rms and lag behaviour of a weak narrow Lorentzian (often insignificant in the full-band power spectrum) all point to a QPO origin, likely corresponding to the Type-C QPO broadly seen in Low-Hard and Hard-Intermediate States during outburst rise.
Our findings support a model in which the QPO is an additive process, rather than a modulation of broadband noise or a multiplicative variability pattern, such as precession-induced modulation. This interpretation has important implications for understanding the geometry and dynamics of accretion flows, suggesting that distinct variability components arise from independent physical mechanisms.
In this talk, I will briefly present the observational evidence and then focus on the implications of these results on spectral-timing studies of accreting compact objects.
| Affiliation | Instituto Argentino de Radioastronomía (IAR-CONICET) |
|---|---|
| fgarcia@iar.unlp.edu.ar |
