Lower-twin-peak quasiperiodic oscillation coherence in x-ray binaries and matter stretched by tides falling onto a compact object

Low mass x-ray binaries (LMXBs), with either a neutron star (NS) or a black hole, show in their power spectra quasiperiodic oscillations (QPOs). Those at highest frequencies show up in pairs and are named twin peak high frequency QPOs (HF QPOs). Their central frequencies are typical of the orbital motion timescale close to the compact object. HF QPOs are believed to carry unique information on the matter moving in the extreme gravitational field around the compact object. In previous works we highlighted the work done by strong tides on clumps of plasma orbiting in the accretion disk as suitable mechanism to produce the HF QPOs. We showed that the upper of the twin peak HF QPOs seen in NS LMXBs could originate from the tidal circularization of the clump's relativistic orbit, while the lower HF QPO could come from the spiraling clump losing orbital energy. Here we focus on the tidal deformation of a magnetized clump of plasma once tides load energy on it. The likely evolution of the shape of the clump after tidal circularization of the orbit and its subsequent orbital evolution are investigated. In atoll NS LMXBs, a subclass of NS LMXBs less luminous than Z NS LMXBs, the lower HF QPO displays a characteristic behavior of its coherence $Q$ versus its central frequency $\nu$. $Q$ keeps increasing over the range of frequencies $\nu\sim$ 600-850 Hz and then drops abruptly at $\nu\sim$ 900 Hz. We note, for the first time, that such behavior is reproduced by magnetized clumps of plasma stretched by strong tides and falling onto the NS on unstable orbits. We emphasize the overall behavior of the lower HF QPO coherence as candidate to disclose the innermost stable bound orbit predicted by the general relativity theory in the strong field regime.

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