Dust Echoes from the Ambient Medium of Gamma‐Ray Bursts

Long gamma-ray bursts (GRBs) are likely associated with the collapse ofmassive stars, which produce dust and are born in dusty environments.Absorption and scattering of ultraviolet/X-ray photons from the prompt, opticalflash and afterglow emission of the GRB produce dust echoes. We performtime-dependent calculations of these echoes, accounting for the evolution ofthe dust grain distribution due to selective grain destruction by the GRBradiation, and for off-axis beaming. We explore cloud configurations ofdiffering density and size -- the echo light curve and spectrum depend on thecloud radius, with larger clouds peaking at longer wavelengths. For a region ~3 pc in size with hydrogen density ~ 1000 per cubic centimeter, the echospectrum peaks at ~ 3.6 microns and ~ 8.8 eV for thermal and scatteredcomponents, respectively. Dust echoes should be detectable with the Very LargeTelescope up to z ~ 0.1, IRAC onboard the Spitzer Space Telescope up to z ~0.2, and NICMOS onboard the Hubble Space Telescope up to z ~ 0.3. Furthermore,the shape of the echo light curve allows one to infer: the jet opening angle;the inclination of the jet axis with respect to the line of sight; the size ofthe dust-emitting region. For sources with symmetric, bipolar jets, dust echoesexhibit two bumps in the light curve, making them easily distinguishable fromthe rebrightening due to an underlying supernova.Comment: 24 pages, 10 figures. Accepted by Ap