GRB 080710: A Narrow, Structured Jet Showing a Late, Achromatic Peak in the Optical and Infrared Afterglow?

We present a possible theoretical interpretation of the observed afterglow emission of long gamma-ray burst GRB 080710. While its prompt GRB emission properties are normal, the afterglow light curves in the optical and infrared bands are exceptional in two respects. One is that the observed light curves of different wavelengths have maximum at the same time, and that the achromatic peak time, 2.2×103 s after the burst trigger, is about an order of magnitude later than typical events. The other is that the observed flux before the peak increases more slowly than theoretically expected so far. Assuming that the angular distribution of the outflow energy is top-hat or Gaussian-shaped, we calculate the observed light curves of the synchrotron emission from the relativistic jets and explore the model parameters that explain the observed data. It is found that a narrowly collimated Gaussian-shaped jet with large isotropic-equivalent energy is the most plausible model for reproducing the observed afterglow behavior. Namely, an off-axis afterglow scenario to the achromatic peak is unlikely. The inferred values of the opening angle and the isotropic equivalent energy of the jet are possibly similar to those of GRB 221009A, but the jet of GRB 080710 has a much smaller efficiency of the prompt gamma-ray emission. Our results indicate a greater diversity of the GRB jet properties than previously thought.