Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase

¹ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: avdhorst@science.uva.nl
² Raman Research Institute, Bangalore 560080, India
³ Joint Astronomy Programme, Indian Institute of Science, Bangalore 560012, India
⁴ Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India
⁵ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE2 7RH, UK
⁶ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁷ NASA/MSFC, NSSTC, VP62, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁸ National Centre for Radio Astrophysics, Post Bag 3, Ganeshkind, Pune 411007, India

Received: 9 June 2007
Accepted: 20 December 2007

Abstract

Context.We explore the physics behind one of the brightest radio afterglows ever, GRB 030329, at late times when the jet is non-relativistic.

Aims.We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements.

Methods.We observed the GRB 030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268-1128 days after the burst. We modeled all the available radio data and derived the physical parameters.

Results.The index of the electron energy distribution is , the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous findings.

Conclusions.Our findings indicate that the blast wave is roughly spherical at , and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB 030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further.