GRB 080928 afterglow imaging and spectro-polarimetry

Context. Among the large variety of astrophysical sources that we can observe, gamma-ray bursts (GRBs) are the most energetic of the whole Universe. Their emission peaks in the γ-ray band, with a duration from a fraction of a second to a few hundred seconds, and is followed by an afterglow covering the whole electromagnetic spectrum. The definition of a general picture describing the physics behind GRBs has always been a compelling task, but the results obtained so far from observations have revealed a puzzling landscape. The lack of a clear, unique paradigm calls for further observations and additional, independent techniques for this purpose. Polarimetry constitutes a very useful example as it allows us to investigate some features of the source such as the geometry of the emitting region and the magnetic field configuration. Aims: To date, only a handful of bursts detected by space telescopes have been accompanied by ground-based spectro-polarimetric follow-up, and therefore such an analysis of more GRBs is of crucial importance in order to increase the sample of bursts with multi-epoch polarisation analysis. In this work, we present the analysis of the GRB 080928 optical afterglow, with observations performed with the ESO-VLT FORS1 instrument. Methods: Starting from raw data taken in the imaging polarimetry (IPOL) and spectro-polarimetry (PMOS) modes, we performed data reduction, followed by the photometric analysis of IPOL data, taken ~14 and ~40 h after the burst detection, and spectroscopy of PMOS data (t ~ 14.95 h). After computing the reduced Stokes parameters Q/I and U/I, which describe the linear polarisation of the emitted radiation, we obtained the polarisation degree for the three observing epochs. Results: We find that the GRB optical afterglow was not significantly polarised on the first observing night. The polarisation degree (P) grew on the following night to a level of P ~ 4.5%, giving evidence of polarised radiation at a 4σ confidence level. The GRB 080928 light curve is not fully consistent with standard afterglow models, making any comparison with polarimetric models partly inconclusive. The most conservative interpretation is that the GRB emission was characterised by a homogeneous jet and was observed at an angle of 0.6 <θobs/θjet < 0.8. Moreover, the non-zero polarisation degree on the second night suggests the presence of a dominant locally ordered magnetic field in the emitting region.