  Twenty-two extragalactic fast X-ray transients (FXTs) have now been
discovered from two decades of Chandra data (analyzing ~259 Ms of data), with
17 associated with distant galaxies (>100 Mpc). Different mechanisms and
progenitors have been proposed to explain their properties; nevertheless, after
analyzing their timing, spectral parameters, host-galaxy properties, luminosity
function, and volumetric rates, their nature remains uncertain. We interpret a
sub-sample of nine FXTs that show a plateau or a fast-rise light curve within
the framework of a binary neutron star (BNS) merger magnetar model. We fit
their light curves and derive magnetar (magnetic field and initial rotational
period) and ejecta (ejecta mass and opacity) parameters. This model predicts
two zones: an orientation-dependent free zone (where the magnetar spin-down
X-ray photons escape freely to the observer) and a trapped zone (where the
X-ray photons are initially obscured and only escape freely once the ejecta
material becomes optically thin). We argue that six FXTs show properties
consistent with the free zone and three FXTs with the trapped zone. This
sub-sample of FXTs has a similar distribution of magnetic fields and initial
rotation periods to those inferred for short gamma-ray bursts (SGRBs),
suggesting a possible association. We compare the predicted ejecta emission fed
by the magnetar emission (called merger-nova) to the optical and near-infrared
upper limits of two FXTs, XRT 141001 and XRT 210423 where contemporaneous
optical observations are available. The non-detections place lower limits on
the redshifts of XRT 141001 and XRT 210423 of z>1.5 and >0.1, respectively. If
the magnetar remnants lose energy via gravitational waves, it should be
possible to detect similar objects with the current advanced LIGO detectors out
to a redshift z<0.03, while future GW detectors will be able to detect them out
to z=0.5.
