  We report on Chandra, RXTE, Swift/BAT and MAXI observations of a ~1 day X-ray
flare and subsequent outburst of a transient X-ray source observed in
October-November 2011 in the globular cluster Terzan 5. We show that the source
is the same as the transient that was active in 2000, i.e., the neutron star
low-mass X-ray binary EXO 1745-248. For the X-ray flare we estimate a 6-11 hr
exponential decay time and a radiated energy of 2-9 x 10^42 erg. These
properties, together with strong evidence of decreasing blackbody temperature
during the flare decay, are fully consistent with what is expected for a
thermonuclear superburst. We use the most recent superburst models and estimate
an ignition column depth of ~10^12 g cm^-2 and an energy release between 0.1-2
x 10^18 erg g^-1, also consistent with expected superburst values. We conclude
therefore that the flare was most probably a superburst. We discuss our results
in the context of theoretical models and find that even when assuming a few
days of low level accretion before the superburst onset (which is more than
what is suggested by the data), the observations of this superburst are very
challenging for current superburst ignition models.
