BVRI Observations of the Optical Afterglow of GRB 990510

We present BVRI observations of the optical counterpart to GRB 990510 obtained with the Las Campanas 1.0 m telescope between 15 and 48 hr after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically t-0.76±0.01 at early times (t 1 day) and t-2.40±0.02 at late times, with the break time at t0 = 1.57 ± 0.03 days. GRB 990510 is the most rapidly fading of the well-documented gamma-ray burst (GRB) afterglows. It is also the first well-observed example of a broadband break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power law with ν-0.61±0.12. However, when the B-band point is dropped from the fit, the power law becomes ν-0.46±0.08, indicating a possible deviation from the power law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at z = 1.62. Broadband break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.