GRB Afterglow Polarimetry: Past, Present and Future

Historically, essentially all interpretative studies about GRB afterglow polarimetry have been based on the cosmological fireball model (26; 33), which we will also use as a reference for our discussion. Afterglow polarization studies have indeed the advantage that different models are often almost indistinguishable in term of radiation output in the optical, but produce markedly distinct predictions about polarization.

In this proceeding, we will briefly review what we have derived by optical afterglow polarimetric observations in Sect. 1.2 and discuss the most recent development in the field in Sect. 1.3. For a deeper discussion about the physical ingredients generating a polarized flux in GRB afterglow radiation one can refer to other proceedings in this volume (15; 17; 6).

We report below what we consider the three most important achievements obtained by afterglow polarimetric observation in GRB research. Generally speaking, two general families of models have been developed to explain why GRB afterglows can be polarized and the time evolution of polarization. One possibility is that the emission originates in causally disconnected regions of highly ordered magnetic field, each producing polarization almost at the maximum degree. (13) predicted a ∼ 10% polarization. If the regions have a statistical distribution of energies, the position angle can be different at various wavelengths. This value is greater than that observed in many GRB afterglows (4) as most of the positive detections so far derived are below ∼ 3%. In an alternative scenario first introduced by (? ) and then developed by (10? ) the magnetic field is ordered in the plane of the shock. In a spherical fireball, such a field configuration would give null polarization, but if a collimated fireball is observed off-axis (as it is most probable), a small degree of polarization would be predicted, with a well defined temporal evolution. Here the ultrarelativistic motion toward the observer and the physical beaming of the outflow are fundamental ingredients.

After a few unfruitful attempts which (14), and not by chance as soon as the first unit of the ESO-VLT become operational, a low although highly significant polarization for the afterglow of GRB 990510 (Fig. 1.1) was successfully detected for the first time (2; 31). This simple observational finding carried already a lot of information. First of all, the detection of polarized flux from a GRB afterglow can and has been considered a clear signature for synchrotron emission, although various alternative explanations indeed exist. In general, the detected polarization (1.7% ± 0.2%) would require emission processes involving particle acceleration. In the external shock phenomenon we have particle acceleration at the shock front and once we consider the ultrarelativistic motion toward the observer and the physical beaming of the outflow some level of polarization in the afterglows is naturally predicted. It is possible to have some degree of polarization adopting other scenarios, however in no case a polarized flux is a natural output of the model, as it is for the cosmological fireball model. To my knowledge, this is still one of the most convincing, although admittedly often unrecorded, observational proof supporting the standard afterglow model.

The detection of varying polarization on time scales comparable to those of the afterglow evolution, immediately implies that the observed polarization is intrinsic to the source and not, for instance, due to scattering against material along the line of sight. The first convincing evidence of time-varying polarization was obtained for GRB 020813 (1; 19), where a decrease of the polarization degree from ∼ 3% down to less than 1% (Fig. 1.2), with constant position angle, was recorded from a few hours to half a day after the burst. Evolution was also singled out in GRB 021004 (18).

The most striking example is however GRB 030329. Due to its relatively small distance, a very high quality dataset was obtained covering more than two weeks (12). Strong, somewhat erratic, variations of the polarization degree and position angle during the afterglow evolution were singled out. Polarization variations occurred on a time scale comparable to that of the afterglow flux variability, offering a direct link between the two phenomena (11) although in the case of GRB 030329 the late-time rise of the supernova (associated to the GRB) component played an important role. (19). The shaded area shows where a break, possibly a jet break, was observed during the optical afterglow evolution (3

…(Full text truncated)…