GRB 090313 AND THE ORIGIN OF OPTICAL PEAKS IN GAMMA-RAY BURST LIGHT CURVES: IMPLICATIONS FOR LORENTZ FACTORS AND RADIO FLARES

We use a sample of 19 gamma-ray bursts (GRBs) that exhibit single-peaked optical light curves to test the standard fireball model by investigating the relationship between the time of the onset of the afterglow and the temporal rising index. Our sample includes GRBs and X-ray flashes for which we derive a wide range of initial Lorentz factors (40 < Γ < 450). Using plausible model parameters, the typical frequency of the forward shock is expected to lie close to the optical band; within this low typical frequency framework, we use the optical data to constrain εe and show that values derived from the early time light-curve properties are consistent with published typical values derived from other afterglow studies. We produce expected radio light curves by predicting the temporal evolution of the expected radio emission from forward and reverse shock components, including synchrotron self-absorption effects at early time. Although a number of GRBs in this sample do not have published radio measurements, we demonstrate the effectiveness of thismethod in the case of Swift GRB 090313, for which millimetric and centimetric observations were available, and conclude that future detections of reverse-shock radio flares with newradio facilities such as the EVLA and ALMA will test the low-frequency model and provide constraints on magnetic models.
A.M. acknowledges funding from the Particle Physics and Astronomy Research Council (PPARC). C.G.M. is grateful for financial support from the Royal Society and Research Councils (UK). A.G. acknowledges founding from the Slovenian Research Agency and from the Centre of Excellence for Space Sciences and Technologies SPACE-SI, an operation partly financed by the European Union, European Regional Development Fund and Republic of Slovenia, Ministry of Higher Education, Science and Technology. I.d.G.-M. is partially supported by Ministerio de Ciencia e Innovaci´on (Spain), grant AYA2008-06189-C03 (including FEDER funds), and by Consejer´ıa de Innovaci´on, Ciencia y Empresa of Junta deAndaluc´ıa (Spain). The Liverpool Telescope is operated by Liverpool John Moores University at the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The Faulkes Telescopes, now owned by Las Cumbres Observatory, are operated with support from the Dill Faulkes Educational Trust. This work is partially based on observations carried out with the IRAM Plateau de Bure Interferometer and observations collected at the German-Spanish Astronomical Center, Calar Alto, jointly operated by the Max-Planck-Institut f¨ur Astronomie Heidelberg and the Instituto de Astrof´ısica de Andaluc´ıa (CSIC). IRAM is supported by INSU/CNRS (France),MPG (Germany), and IGN (Spain). We thank Calar Alto Observatory for allocation of director’s discretionary time to this program. We also thank M.R Zapatero-Osorio for the acquisition and reduction of NOT data. The research of J.G. and A.J.C.T. is supported by the Spanish programmes AYA2007-63677, AYA2008-03467/ESP, and AYA2009-14000-C03-01. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.