The recent gamma ray burst , GRB 990123 , has an absorption redshift z _ { s } = 1.60 , implying an apparent energy E \geq 3 \times 10 ^ { 54 } erg , and a peak luminosity L _ { { max } } \geq 6 \times 10 ^ { 53 } erg s ^ { -1 } , assuming isotropic emission .
This energy is ten times larger than hitherto measured and in excess of the rest mass of the sun .
Optical observations have revealed an associated galaxy displaced from the line of sight by \sim 0.6 ^ { \prime \prime } .
This raises the possibility that the burst is enhanced by gravitational lensing .
We argue that existing observations probably only allow magnifications \mu > 400 if the galaxy is at z _ { d } = 1.60 and the burst originates at much higher redshift .
It should be possible to exclude this possibility by examining the burst time structure .
If , as we anticipate , multiple imaging can be excluded , GRB 990123 remains the most intrinsically luminous event yet observed in its entirety .