Early-time VLA Observations and Broadband Afterglow Analysis of the Fermi/LAT Detected GRB 130907A
We present multi-wavelength observations of the hyper-energetic gamma-ray burst (GRB) 130907A, a Swift-discovered burst with early radio observations starting at ≈4 hr after the γ-ray trigger. GRB 130907A was also detected by the Fermi/LAT instrument and at late times showed a strong spectral evolution in X-rays. We focus on the early-time radio observations, especially at >10 GHz, to attempt to identify reverse shock signatures. While our radio follow-up of GRB 130907A ranks among the earliest observations of a GRB with the Karl G. Jansky Very Large Array, we did not see an unambiguous signature of a reverse shock. While a model with both reverse and forward shock can correctly describe the observations, the data is not constraining enough to decide upon the presence of the reverse-shock component. We model the broadband data using a simple forward-shock synchrotron scenario with a transition from a wind environment to a constant density interstellar medium (ISM) in order to account for the observed features. Within the confines of this model, we also derive the underlying physical parameters of the fireball, which are within typical ranges except for the wind density parameter (A*), which is higher than those for bursts with wind-ISM transition, but typical for the general population of bursts. We note the importance of early-time radio observations of the afterglow (and of well-sampled light curves) for unambiguously identifying the potential contribution of the reverse shock.
© 2015 The American Astronomical Society. Received 2014 November 26; accepted 2015 July 20; published 2015 August 26. We thank Phil Evans and Bing Zhang for valuable discussions, and the anonymous referee for useful comments. P.V. acknowledges support from Fermi grant NNM11AA01A and OTKA NN 111016 grant. A.C. acknowledges partial support from the NASA-Swift GI program via grants 13-SWIFT13-0030 and 14-SWIFT14-0024. Support for S.B.C. was provided by NASA through the Fermi grant NNH13ZDA001N. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.
Published - Veres_2015.pdf