Early-time polarized optical light curve of GRB 131030A
We report the polarized optical light curve of a gamma-ray burst afterglow obtained using the RoboPol instrument. Observations began 655 s after the initial burst of gamma-rays from GRB 131030A, and continued uninterrupted for 2 h. The afterglow displayed a low, constant fractional linear polarization of p = (2.1 ± 1.6) per cent throughout, which is similar to the interstellar polarization measured on nearby stars. The optical brightness decay is consistent with a forward-shock propagating in a medium of constant density, and the low polarization fraction indicates a disordered magnetic field in the shock front. This supports the idea that the magnetic field is amplified by plasma instabilities on the shock front. These plasma instabilities produce strong magnetic fields with random directions on scales much smaller than the total observable region of the shock, and the resulting randomly-oriented polarization vectors sum to produce a low net polarization over the total observable region of the shock.
Additional Information© 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2014 September 7. Received 2014 September 5; in original form 2014 July 11. First published online October 14, 2014. The RoboPol project is a collaboration between Caltech in the USA, MPIfR in Germany, Toruń Centre for Astronomy in Poland, the University of Crete/FORTH in Greece, and IUCAA in India. The University of Crete group acknowledges support by the 'RoboPol' project, which is implemented under the 'Aristeia' Action of the 'Operational Programme Education and Lifelong Learning' and is co-funded by the European Social Fund (ESF) and Greek National Resources, and by the European Commission Seventh Framework Programme (FP7) through grants PCIG10-GA-2011-304001 'JetPop' and PIRSES-GA-2012-31578 'EuroCal'. This research was supported in part by NASA grant NNX11A043G and NSF grant AST-1109911, and by the Polish National Science Centre, grant number 2011/01/B/ST9/04618. KT acknowledges support by the European Commission Seventh Framework Programme (FP7) through the Marie Curie Career Integration Grant PCIG-GA-2011-293531 'SFOnset'. MB acknowledges support from the International Fulbright Science and Technology Award. IM and SK are supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. TH was supported by the Academy of Finland project number 267324. This research made use of ASTROPY, http://www.astropy.org, a community-developed core PYTHON package for Astronomy (Astropy Collaboration et al. 2013). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Published - MNRAS-2014-King-L114-8.pdf
Submitted - 1409.2417v2.pdf