Radio observations of GRB 100418a: Test of an energy injection model explaining long-lasting GRB afterglows

Abstract

We present the results of our radio observational campaign on GRB 100418a, for which we used the Australia Telescope Compact Array (ATCA), Very Large Array (VLA) and the Very Long Baseline Array (VLBA). GRB 100418a was a peculiar GRB with unusual X-ray and optical afterglow profiles featuring a plateau phase with a very shallow rise. This observed plateau phase was believed to be due to a continued energy injection mechanism which powered the forward shock, giving rise to an unusual and long-lasting afterglow. The radio afterglow of GRB 100418a was detectable several weeks after the prompt emission. We conducted long-term monitoring observations of the afterglow and attempted to test the energy injection model advocating that the continuous energy injection is due to shells of material moving at a wide range of Lorentz factors. We obtained an upper limit of $\gamma$ $<$ 7 for the expansion rate of the GRB 100418a radio afterglow, indicating that the range-of-Lorentz factor model could only be applicable for relatively slow moving ejecta. A preferred explanation could be that continued activity of the central engine may have powered the long-lasting afterglow.

8 pages, 3 figures, accepted for publication in ApJ

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• Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions
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• The Redshift Dependence of the Radio Flux of Gamma-Ray Bursts and Their Host Galaxies
• Diagnosing Circumburst Environment with Multiband Gamma-Ray Burst Radio Afterglows
• Discovery and Analysis of Afterglows from Poorly Localised GRBs with the Gravitational-wave Optical Transient Observer (GOTO) All-sky Survey
• The GROND GRB sample: II. Fireball parameters for four GRB afterglows