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

We present the results of our radio observational campaign of gamma-ray burst (GRB) 100418a, for which we used the Australia Telescope Compact Array, the Very Large Array, and the Very Long Baseline Array. 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 that 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 γ < 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.