A Characteristic Dense Environment or Wind Signature in Prompt GRB Afterglows

We discuss the effects of synchrotron self-absorption in the prompt emission from the reverse shock of GRB afterglows in a dense environment, such as the wind of a stellar progenitor or a dense ISM in early galaxies. We point out that, when synchrotron losses dominate over inverse Compton losses, the higher selfabsorption frequency in a dense environment implies a bump in the reverse shock emission spectrum, which can result in a more complex optical/IR light curve than previously thought. This bump is prominent especially if the burst ejecta is highly magnetized. In the opposite case of low magnetization, inverse Compton losses lead to a prompt X-ray flare. These effects give a possible new diagnostic for the magnetic energy density in the fireball, and for the presence of a dense environment. Subject headings: gamma rays: bursts — shock waves — radiation mechanisms: thermal