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 afterglowemission of long gamma-ray burst GRB 080710. While its prompt GRB emissionproperties are normal, the afterglow light curves in the optical and infraredbands are exceptional in two respects. One is that the observed light curves ofdifferent wavelengths have maximum at the same time, and that the achromaticpeak time, $2.2\times10^3$ s after the burst trigger, is about an order ofmagnitude later than typical events. The other is that the observed flux beforethe peak increases more slowly than theoretically expected so far. Assumingthat the angular distribution of the outflow energy is top-hat orGaussian-shaped, we calculate the observed light curves of the synchrotronemission from the relativistic jets and explore the model parameters thatexplain the observed data. It is found that a narrowly collimatedGaussian-shaped jet with large isotropic-equivalent energy is the mostplausible model for reproducing the observed afterglow behavior. Namely, anoff-axis afterglow scenario to the achromatic peak is unlikely. The inferredvalues of the opening angle and the isotropic equivalent energy of the jet arepossibly similar to those of GRB 221009A, but the jet of GRB 080710 has a muchsmaller efficiency of the prompt gamma-ray emission. Our results indicate agreater diversity of the GRB jet properties than previously thought.