GRB 080928 afterglow imaging and spectro-polarimetry
Journal
Date Issued
2022
Author(s)
Brivio, R.
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•
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Wiersema, K.
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Maund, J. R.
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•
•
Abstract
Among the large variety of astrophysical sources that we can observe,
gamma-ray bursts (GRBs) are the most energetic of the whole Universe. 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. 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. 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 $P \sim$ 4.5%, giving evidence of polarised radiation at a
4 $\sigma$ 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 $< \theta_{obs}/\theta_{jet} <$ 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.
Volume
666
Start page
A179
Issn Identifier
0004-6361
Rights
open.access
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