Abstract
[Abridged] A number of phenomena have been observed in GRB afterglows that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well calibrated dataset of a typical afterglow to serve as a benchmark system for modelling afterglow polarisation behaviour. In particular, our dataset of the afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high quality optical and near-infrared broadband light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarisation varies between 0 and 3%, with both long and short time scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarised component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R and Ks band polarimetry shows that dust induced polarisation in the host galaxy is likely negligible.
Highlights
Collimated outflows in the form of jets are ubiquitous, from active galactic nucleus jets driven by accretion of material by supermassive black holes in galactic centres to those associated with stellar sources such as X-ray binaries and galactic microquasars
Follow-up observations of large numbers of gamma-ray burst (GRB) afterglows, facilitated by the Swift satellite, have produced a large sample of spectral energy distributions and light curves, from which their basic micro- and macro-physical parameters can in principle be derived
If we instead use the empirical conversion from [O II] line luminosity to SFT derived by Savaglio et al (2009) for GRB host galaxies, we find star formation rate (SFR) ([O II]) ∼1.6 M per year
Summary
Collimated outflows in the form of jets are ubiquitous, from active galactic nucleus jets driven by accretion of material by supermassive black holes in galactic centres to those associated with stellar sources such as X-ray binaries and galactic microquasars. A exciting view of the fundamental physics of jet sources is offered by gamma-ray bursts (GRBs), which form the extreme end of the energy and Lorentz factor parameter space. Meszaros & Rees 1997), the resulting broad-band emission detected from X-ray energies through to radio waves – the so-called afterglow – is explained by the relativistic ejecta colliding with the surrounding circumburst medium. The afterglow radiation we detect is consistent with a synchrotron emission The macroscopic properties of shocks are largely understood, and the dynamics of the shock created when the relativistic jet hits the circumstellar matter can be written down in terms of the explosion energy, the density (and density gradient) of the medium into which the shock ploughs and the composition of the shocked material. Outstanding questions remain on the nature of the microphysics: how are the relativistic particles that radiate the observed emission accelerated? Where does the magnetic field in the shocked region come from and what is its structure?
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