We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint (mR = 27.5mag, zph ~2.2) host in the CANDELS survey. The X-ray event is comprised of 115+12 -11 net 0.3-7.0 keV counts, with a light curve characterized by an ~100 s rise time, a peak 0.3-10 keV flux of ~5 × 10-12 erg s-1 cm-2 and a power-law decay time slope of -1.53 ± 0.27. The average spectral slope is Γ = 1.43+0.23 -0.13, with no clear spectral variations. The X-ray and multiwavelength properties effectively rule out the vast majority of previously observed highenergy transients. A few theoretical possibilities remain: an 'orphan' X-ray afterglow from an off-axis short-duration gamma-ray burst (GRB) with weak optical emission, a low-luminosity GRB at high redshift with no prompt emission below ~20 keV rest frame, or a highly beamed tidal disruption event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.
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