Early optical afterglow light curves of neutron-fed gamma-ray bursts

The neutron component is likely an inevitable ingredient of a gamma-ray burst (GRB) baryonic fireball, in essentially all progenitor scenarios. The suggestion that the neutron composition may alter the early afterglow behavior has been proposed, but there is no detailed calculation so far. In this paper, within the popular internal shock scenario of GRBs, we calculate the early optical afterglow light curves of a neutron-fed GRB fireball for different assumed neutron fractions in the fireball and for both ISM- and wind-interaction models. The cases for both long and short GRBs are considered. We show that as long as the neutron fraction is significant (e. g., the number of neutrons is comparable to that of protons), rich afterglow signatures would show up. For a constant-density (ISM) model, a neutron-rich early afterglow is characterized by a slowly rising light curve followed by a sharp rebrightening bump caused by a collision between the leading neutron decay trail ejecta and the trailing ion ejecta. For a massive star stellar wind model, the neutron-rich early afterglow shows an extended plateau lasting for about 100 s before the light curve starts to decay. The plateau is mainly attributed to the emission from the unshocked neutron decay trail. When the overlapping of the initial prompt gamma-rays with the shocks and the trail is important, as is common for the wind model and is also possible in the ISM model under some conditions, the IC cooling effect suppresses the very early optical afterglow significantly, making the neutron-fed signature dimmer. For short GRBs powered by compact star mergers, a neutron decay-induced steplike rebrightening is predicted, although the amplitude is not large. All these neutron-fed signatures are likely detectable by the Ultraviolet Optical Telescope (UVOT) on board the Swift observatory if GRB fireballs are indeed baryonic and neutron-rich. Close monitoring of early afterglows from tens to thousands of seconds, when combined with detailed theoretical modeling, could be used to potentially diagnose the existence of the neutron component in GRB fireballs.