Will GRB 990123 perform an encore?

The recent gamma‐ray burst GRB 990123 has an absorption redshift z s =1.60, implying an apparent energy E ≥ 3 × 10 54 erg, and a peak luminosity L max ≥ 6 × 10 53 erg s ‐1 , assuming isotropic emission. This energy is 10 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 ∼ 0.6 arcsec. This raises the possibility that the burst is enhanced by gravitational lensing. We argue that existing observations probably only allow magnifications μ >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. Although we anticipate that multiple imaging can be excluded in GRB 990123, our analysis should be generally applicable.