We report the discovery by the \Swift hard X-ray monitor of the transient source \Swift J2058.4+0516 ( \event ) .
Our multi-wavelength follow-up campaign uncovered a long-lived ( duration \gtrsim months ) , luminous X-ray ( L _ { \mathrm { X,iso } } \approx 3 \times 10 ^ { 47 } erg s ^ { -1 } ) and radio ( \nu L _ { \nu, \mathrm { iso } } \approx 10 ^ { 42 } erg s ^ { -1 } ) counterpart .
The associated optical emission , however , from which we measure a redshift of 1.1853 , is relatively faint , and this is not due to a large amount of dust extinction in the host galaxy .
Based on numerous similarities with the recently discovered GRB 110328A / \Swift J164449.3+573451 ( Sw J1644+57 ) , we suggest that \event may be the second member of a new class of relativistic outbursts resulting from the tidal disruption of a star by a supermassive black hole .
If so , the relative rarity of these sources ( compared with the expected rate of tidal disruptions ) implies that either these outflows are extremely narrowly collimated ( \theta < 1 ^ { \circ } ) , or only a small fraction of tidal disruptions generate relativistic ejecta .
Analogous to the case of long-duration gamma-ray bursts and core-collapse supernovae , we speculate that rapid spin of the black hole may be a necessary condition to generate the relativistic component .
Alternatively , if powered by gas accretion ( i.e. , an active galactic nucleus [ AGN ] ) , \event would seem to represent a new mode of variability in these sources , as the observed properties appear largely inconsistent with known classes of AGNs capable of generating relativistic jets ( blazars , narrow-line Seyfert 1 galaxies ) .