Optical properties of GRB afterglows

Optical emission was detected for 24 GRBs with brightnesses ranging from R=10 to 22mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t=100 to ~106 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths.

While 14/24 GRBs can be modeled with the forward-shock model, explaining the remaining 10 is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power law, may be due to additional emission from the central engine. Thirty-nine GRBs in our sample were not detected and have deep upper limits (R<22mag) at early time.