Optical flashes, reverse shocks and magnetization

TL;DR

This paper reviews the occurrence of optical flashes from reverse shocks in GRB afterglows, highlighting the role of magnetization and suggesting flashes may occur at lower wavelengths like radio.

Contribution

It provides a comprehensive analysis of reverse shock signatures in optical afterglows and discusses how magnetization influences their presence and detectability.

Findings

Optical flashes are not always observed in early GRB afterglows.

Magnetization of the fireball affects reverse shock emission.

Reverse shocks may produce signals at radio wavelengths instead of optical.

Abstract

Despite the pre-Swift expectation that bright optical flashes from reverse shocks would be prevalent in early-time afterglow emission, rapid response observations show this not to be the case. Although very bright at early times, some GRBs such as GRB 061007 and GRB 060418, lack the short-lived optical flash from the reverse shock within minutes after the GRB. In contrast, other optical afterglows, such as those of GRB 990123, GRB 021211, GRB 060111B, GRB 060117, GRB 061126, and recently GRB 080319B, show a steep-to-flat transition within first 10^3 s typical of a rapidly evolving reverse + forward shock combination. We review the presence and absence of the reverse shock components in optical afterglows and discuss the implications for the standard model and the magnetization of the fireball. We show that the previously predicted optical flashes are likely to occur at lower wavelengths, perhaps as low as radio wavelengths and, by using the case of GRB 061126 we show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a key physical parameter.