SDSS J124602.54 + 011318.8: A Highly Luminous Optical Transient at z = 0.385*

We report the discovery of a highly luminous optical transient (OT), SDSS J124602.54 + 011318.8, associated with a galaxy at a redshift of 0.385. In this paper, we consider the possibility that the OT may be a gamma-ray burst (GRB) afterglow. Three sets of images and two sets of spectra were obtained as part of the normal operations of the Sloan Digital Sky Survey (SDSS). In the first two image sets, observed two nights apart, the object appears as a point source at r* ≈ 17. The third image set, observed about 410 days later, shows an extended source which is more than 2.5 mags fainter. The spectra were observed about 400 and 670 days after the first two image sets, and both show an apparently normal galaxy at a redshift of 0.385. Associating the OT with the galaxy, the absolute magnitude was M = -24.8, which is over 4 mag brighter than the most luminous supernova ever measured. The spectral energy distributions of the galaxy-subtracted OT derived from the first two image sets are well-fitted by single power laws with indices of βν = -0.92 and -1.29, respectively, similar to most GRB afterglows. Based upon the luminosity of the OT, nondetections in contemporaneous ROTSE I images and the change in spectral slope, the OT, if an afterglow, was likely discovered early during a plateau or slowly fading phase. The discovery of a GRB afterglow at this stage of the SDSS is consistent with expectations, but only if the optical emission is much less strongly beamed than the gamma rays. We emphasize that other explanations for the OT cannot be ruled out; a recent follow-up study by Gal-Yam et al. (2002) provides strong evidence that this source is in fact an unusual active galactic nucleus (AGN).