  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.
