Microphysics and dynamics of the gamma-ray burst 121024A

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: kvarela@mpe.mpg.de
² Excellence Cluster Universe, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
³ Astrophysics Data System, Harvard-Smithonian Center for Astrophysics Garden St. 60, Cambridge, MA 02138, USA
⁴ European Southern Observatory, Alonso de Córdoba 3107, Vitacura, Casilla 19001 Santiago 19, Chile
⁵ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
⁶ Technische Universität München, Physik Dept., James-Franck-Str., 85748 Garching, Germany
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ European Southern Observatory, Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁰ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 128 00 Prague 2, Czech Republic
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² INAF–IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy

Received: 3 April 2015
Accepted: 1 February 2016

Abstract

Aims. The aim of the study is to constrain the physics of gamma-ray bursts (GRBs) by analysing the multi-wavelength afterglow data set of GRB 121024A that covers the full range from radio to X-rays.

Methods. Using multi-epoch broad-band observations of the GRB 121024A afterglow, we measured the three characteristic break frequencies of the synchrotron spectrum. We used six epochs of combined XRT and GROND data to constrain the temporal slopes, the dust extinction, the X-ray absorption, and the spectral slope with high accuracy. Two more epochs of combined data from XRT, GROND, APEX, CARMA, and EVLA were used to set constraints on the break frequencies and therefore on the micro-physical and dynamical parameters.

Results. The XRT and GROND light curves show a simultaneous and achromatic break at around 49 ks. As a result, the crossing of the synchrotron cooling break is no suitable explanation for the break in the light curve. The multi-wavelength data allow us to test two plausible scenarios explaining the break: a jet break, and the end of energy injection. The jet-break scenario requires a hard electron spectrum, a very low cooling break frequency, and a non-spreading jet. The energy injection avoids these problems, but requires ϵ_e > 1 (k = 2), spherical outflow, and ϵ_B < 10^-9.

Conclusions. In light of the extreme microphysical parameters required by the energy-injection model, we favour a jet-break scenario where ν_m < ν_sa to explain the observations. This scenario gives physically meaningful microphysical parameters, and it also naturally explains the reported detection of linear and circular polarisation.