Compton echoes from gamma-ray bursts

Recent observations of gamma-ray bursts (GRBs) have provided growing eviden ce for collimated outflows and emission and strengthened the connection bet ween GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard photon pulse will propagate in the preburst, dense environment. Circu mstellar material will Compton scatter the prompt GRB radiation and give ri se to a reflection echo. We calculate luminosities, spectra, and light curv es of such Compton echoes in a variety of emission geometries and ambient g as distributions and show that the delayed hard X-ray flash from a pulse pr opagating into a red supergiant wind could be detectable by Swift out to z similar to 0.2. Independently of the gamma-ray spectrum of the prompt burst , reflection echoes will typically show a high-energy cutoff between m(e)c( 2)/2 and m(e)c(2) because of Compton downscattering. At fixed burst energy per steradian, the luminosity of the reflected echo is proportional to the beaming solid angle, Omega(b), of the prompt pulse, while the number of bri ght echoes detectable in the sky above a fixed limiting flux increases as O mega(b)(1/2), i.e., it is smaller in the case of more collimated jets. The lack of an X-ray echo at about 1 month delay from the explosion poses sever e constraints on the possible existence of a lateral GRB jet in SN 1987A. T he late r-band afterglow observed in GRB 990123 is fainter than the optical echo expected in a dense red supergiant environment from an isotropic prom pt optical flash. Significant MeV delayed emission may be produced through the bulk Compton (or Compton drag) effect resulting from the interaction of the decelerating fireball with the scattered X-ray radiation.