Gamma-ray burst dust echoes revisited: Expectations at early times

Gamma-ray burst (GRB) dust echoes were first proposed as an alternative explanation for the supernova-like (SN-like) components to the afterglows of GRB 980326 and GRB 970228. However, the spectroscopic identification of Type Ic SN 2003dh associated with GRB 030329, as well as the identification of SN-like components of the afterglows of other GRBs, appears to have confirmed the GRB/ SN paradigm. However, the likely progenitors of Type Ic SNe are Wolf-Rayet WC stars, and late- type WC stars have been observed to be surrounded by dust at a distance of 10(14)-10(15) cm from the star. Consequently, we revisit the possibility of GRB dust echoes, not on a timescale of weeks after the burst but on a timescale of minutes to hours. We find that if the optical flash is sufficiently bright and the jet sufficiently wide, GRB afterglows may be accompanied by chromatic variations on this timescale. From these signatures, model parameters such as the inner radius of the dust distribution and the initial opening angle of the jet may be deduced. With rapid and regular localizations of GRBs by HETE-2, INTEGRAL, and now Swift, and new and improved robotic telescope systems, these early-time GRB dust echoes may soon be detected. We describe in greater detail one such robotic telescope system, called PROMPT, which the University of North Carolina at Chapel Hill is building at the Cerro Tololo Inter-American Observatory.