2012MNRAS.426..927A

We report on Chandra, Rossi X-ray Timing Explorer (RXTE), Swift/Burst Alert Telescope (BAT) and MAXI observations of an X-ray flare of ∼ 1d and subsequent outburst of a transient X-ray source observed in 2011 October-November 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 6-11h exponential decay time and a radiated energy of 2-9 x 10⁴²erg. 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 ≈ 10¹²g/cm2 and an energy release between 0.1 x 10¹⁸ and 2 x 10¹⁸ergg^–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.