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Low mass x-ray binaries (LMXBs), with either a neutron star (NS) or a blackhole, show in their power spectra quasiperiodic oscillations (QPOs). Those athighest frequencies show up in pairs and are named twin peak high frequencyQPOs (HF QPOs). Their central frequencies are typical of the orbital motiontimescale close to the compact object. HF QPOs are believed to carry uniqueinformation on the matter moving in the extreme gravitational field around thecompact object. In previous works we highlighted the work done by strong tideson clumps of plasma orbiting in the accretion disk as suitable mechanism toproduce the HF QPOs. We showed that the upper of the twin peak HF QPOs seen inNS LMXBs could originate from the tidal circularization of the clump'srelativistic orbit, while the lower HF QPO could come from the spiraling clumplosing orbital energy. Here we focus on the tidal deformation of a magnetizedclump of plasma once tides load energy on it. The likely evolution of the shapeof the clump after tidal circularization of the orbit and its subsequentorbital evolution are investigated. In atoll NS LMXBs, a subclass of NS LMXBsless luminous than Z NS LMXBs, the lower HF QPO displays a characteristicbehavior of its coherence $Q$ versus its central frequency $\nu$. $Q$ keepsincreasing over the range of frequencies $\nu\sim$ 600-850 Hz and then dropsabruptly at $\nu\sim$ 900 Hz. We note, for the first time, that such behavioris reproduced by magnetized clumps of plasma stretched by strong tides andfalling onto the NS on unstable orbits. We emphasize the overall behavior ofthe lower HF QPO coherence as candidate to disclose the innermost stable boundorbit predicted by the general relativity theory in the strong field regime.

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