We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from \approx 150 s to \approx 163 days after the burst , comprising the first joint ALMA-VLA-GMRT observations of a gamma-ray burst ( GRB ) afterglow .
The radio and mm-band data reveal a distinct signature at \approx 3.9 days , which we interpret as reverse shock ( RS ) emission .
Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock ( FS ) components .
We perform detailed modeling of the full multiwavelength data set , using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission .
We uncover and account for all degeneracies in the model parameters .
The joint RS-FS modeling reveals a weakly magnetized ( \sigma \approx 3 \times 10 ^ { -3 } ) , mildly relativistic RS , from which we derive an initial bulk Lorentz factor of \Gamma _ { 0 } \approx 103 for the GRB jet .
Our results support the hypothesis that low-density environments are conducive to the observability of RS emission .
We compare our observations to other events with strong RS detections , and find a likely observational bias selecting for longer lasting , non-relativistic reverse shocks .
We present and begin to address new challenges in modeling posed by the present generation of comprehensive , multi-frequency data sets .