Polarization of Gamma–Ray Burst Optical and Near-Infrared Afterglows

Gamma–Ray Burst afterglow polarization measurements, in spite of their intrinsic difficulties, have been carried out for a number of events that begins to be adequate to draw some general statistical conclusions. Although the presence of some degree of intrinsic polarization seems to be well established, there are still open problems regarding the polarization time evolution and the possible contribution of polarization induced by dust in the host galaxies. 1. Why is polarimetry important for GRB afterglows? Polarization from astrophysical sources is a typical signature for a number of physical phenomena (di Serego et al. 1997). In the context of gamma–ray burst (GRB) physics, the simple detection of some degree of polarization from an optical afterglow (Covino et al. 1999, Wijers et al. 1999; Fig. 1) has always been considered a clear signature for synchrotron emission. Time and wavelength variation of the polarization degree and position angle can be powerful probes for the physics and the dynamics of the expanding fireball and of the GRB environment. 1.1. How polarization can be produced in GRB afterglows As a general rule, to produce observable polarization, some degree of anisotropy is necessarily required. Two general families of models have been developed to explain the level of polarization and its time evolution in GRB optical afterglows. One possibility is that the emission originates in causally disconnected regions of highly ordered magnetic field, each producing polarization almost at the maximum degree (Gruzinov & Waxman 1999; Gruzinov 1999; Medvedev & Loeb 1999; Loeb & Perna 1998). The observed polarization is then lowered by averaging over the unresolved source. Gruzinov & Waxman (1999) predicted a ∼ 10% polarization. If the regions have a statistical distribution of energies, the