  Neutron star-neutron star mergers are known to be associated with short
gamma-ray bursts. If the neutron star equation of state is sufficiently stiff,
at least some of such mergers will leave behind a supramassive or even a stable
neutron star that spins rapidly with a strong magnetic field (i.e., a
magnetar). Such a magnetar signature may have been observed as the X-ray
plateau following a good fraction (up to 50%) of short gamma-ray bursts, and it
has been expected that one may observe short gamma-ray burst-less X-ray
transients powered by double neutron star mergers. A fast X-ray transient
(CDF-S XT1) was recently found to be associated with a faint host galaxy whose
redshift is unknown. Its X-ray and host-galaxy properties allow several
possibleexplanations including a short gamma-ray burst seen off axis, a
low-luminosity gamma-ray burst at high redshift, or a tidal disruption event
involving an intermediate mass black hole and a white dwarf. Here we report a
second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift
z = 0.738. The light curve is fully consistent with being powered by a
millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its
star-forming host galaxy with a moderate offset from the galaxy center, as
short bursts often do. The estimated event rate density of similar X-ray
transients, when corrected to the local value, is consistent with the double
neutron star merger rate density inferred from the detection of GW170817.
