Early optical afterglow lightcurves of neutron-fed Gamma-ray bursts

Abstract

In this paper, within the popular internal shock scenario of GRBs, we calculate the early optical afterglow lightcurves 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 lightcurve followed by a sharp re-brightening bump caused by 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 seconds before the lightcurve 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 step-like re-brightening 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 {\em Swift} observatory if GRB fireballs are indeed baryonic and neutron-rich. Close monitoring of early afterglows from 10s to 1000s of seconds, when combined with detailed theoretical modeling, could be used to potentially 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