RJK band observations of the optical afterglow of GRB 991216

We present near-infrared and optical observations of the afterglow to the gamma-ray burst 991216 obtained with the Fred L. Whipple Observatory 1.2 m telescope and the University of Hawaii 2.2 m telescope. The observations range from 15 hours to 3.8 days after the burst. The temporal behavior of the data is well described by a single power-law decay t(-1.36+/-0.04), independent of wavelength. The optical spectral energy distribution, which is corrected for significant Galactic reddening of E(B-V) = 0.626, is well fitted by a single power law with v(-0.58+/-0.08). Combining the IR/optical observations with a Chandra X-Ray Observatory measurement gives a spectral index of -0.8 +/- 0.1 in the synchrotron cooling regime. A comparison between the spectral and temporal power-law indices suggests that a jet is a better match to the observations than a simple spherical shock.