Early Optical Afterglow Light Curves of Neutron‐fed Gamma‐Ray Bursts

In this paper, within the popular internal shock scenario of GRBs, wecalculate the early optical afterglow lightcurves of a neutron-fed GRB fireballfor different assumed neutron fractions in the fireball and for both ISM- andwind-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 ofneutrons is comparable to that of protons), rich afterglow signatures wouldshow up. For a constant density (ISM) model, a neutron-rich early afterglow ischaracterized by a slowly rising lightcurve followed by a sharp re-brighteningbump caused by collision between the leading neutron decay trail ejecta and thetrailing ion ejecta. For a massive star stellar-wind model, the neutron-richearly afterglow shows an extended plateau lasting for about 100 seconds beforethe lightcurve starts to decay. The plateau is mainly attributed to theemission from the unshocked neutron decay trail. When the overlapping of theinitial prompt $\gamma-$rays with the shocks and the trail is important, as iscommon for the wind model and is also possible in the ISM model under someconditions, the IC cooling effect suppresses the very early optical afterglowsignificantly, making the neutron-fed signature dimmer. For short GRBs poweredby compact star mergers, a neutron-decay-induced step-like re-brightening ispredicted, although the amplitude is not large. All these neutron-fedsignatures are likely detectable by the Ultraviolet Optical Telescope (UVOT) onboard the {\em Swift} observatory if GRB fireballs are indeed baryonic andneutron-rich. Close monitoring of early afterglows from 10s to 1000s ofseconds, when combined with detailed theoretical modeling, could be used topotentially diagnose the existence of the neutron component in GRB fireballs.Comment: 17 pages (6 figures), accepted for publication in ApJ. Several figures are revised, the IC cooling due to the prompt gamma-rays overlap with the shocked regions (stellar wind model) has been taken into account. The possible evidence for the neutron-rich internal shocks (i.e., the prompt optical and near IR flash accompanying GRB 041219a) has been mentioned briefl