Early Optical Polarization of a Gamma-Ray Burst Afterglow

Alessandro Monfardini (1) , Carole Mundell (2) , I. A. Steele (3) , Robert J. Smith , S. Kobayashi , A. Melandri , C. Guidorzi , A. Gomboc , Chris Mottram , D. Clarke , D. Carter , David Bersier (2)

1 NEEL - HELFA - Hélium : du fondamental aux applications
2 ARI - Astrophysics Research Institute [Liverpool]
3 Liverpool JMU

We report the optical polarization of a Gamma Ray Burst afterglow, obtained 203 s after the initial burst of rays from GRB 060418, using a ring polarimeter on the robotic Liverpool Telescope. Our robust (2 sigma) upper limit on the percentage of polarization, less than 8%, coincides with the fireball deceleration time at the onset of the afterglow. The combination of the rate of decay of the optical brightness and the low polarization at this critical time constrain standard models of GRB ejecta, ruling out the presence of a large-scale ordered magnetic field in the emitting region.

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

Alessandro Monfardini, Carole Mundell, I. A. Steele, Robert J. Smith, S. Kobayashi, et al.. Early Optical Polarization of a Gamma-Ray Burst Afterglow. Science, 2007, 315 (5820), pp.1822-1824. ⟨10.1126/science.1138484⟩. ⟨hal-00137180⟩