2001ApJ...546..117R

We present near-infrared observations of the counterpart of GRB 000301C. The K' filter (2.1 µm) light curve shows a well-sampled break in the decay slope at t~3.5 days postburst. The early time slope is very shallow (~-0.1), while the late time slope is steep (-2.2). Comparison with the optical (R-band) light curve shows marginally significant differences, especially in the early time decay slope (which is steeper in the optical) and the break time (which occurs later in the optical). This is contrary to the general expectation that light-curve breaks should either be achromatic (e.g., for breaks due to collimation effects) or should occur later at longer wavelengths (for most other breaks). The observed color variations might be intrinsic to the afterglow, or might indicate systematic errors of ≳0.08 mag in all fluxes. Even if the break is achromatic, we argue that its sharpness poses difficulties for explanations that depend on collimated ejecta. The R light curve shows further signs of fairly rapid variability (a bump, steep drop, and plateau) that are not apparent in the K' light curve. In addition, by combining the IR-optical-UV data with millimeter and radio fluxes, we are able to constrain the locations of the self-absorption break and cooling break and to infer the location of the spectral peak at t=3 days, f_ν~3.4 mJy at ν~10¹² Hz. Using the multiwavelength spectral energy distribution, we are able to constrain the blast wave energy, which was E≳3x10⁵³ ergs if the explosion was isotropic. This implies a maximum gamma-ray production efficiency of ∼0.15 for GRB 000301C.