Compton echoes from gamma-ray bursts

Recent observations of γ-ray bursts (GRBs) have provided growing evidence for collimated outflows and emission and strengthened the connection between GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard photon pulse will propagate in the preburst, dense environment. Circumstellar material will Compton scatter the prompt GRB radiation and give rise to a reflection echo. We calculate luminosities, spectra, and light curves of such Compton echoes in a variety of emission geometries and ambient gas distributions and show that the delayed hard X-ray flash from a pulse propagating into a red supergiant wind could be detectable by Swift out to z ∼ 0.2. Independently of the γ-ray spectrum of the prompt burst, reflection echoes will typically show a high-energy cutoff between mec2/2 and mec2 because of Compton downscattering. At fixed burst energy per steradian, the luminosity of the reflected echo is proportional to the beaming solid angle, Ωb, of the prompt pulse, while the number of bright echoes detectable in the sky above a fixed limiting flux increases as Ω1/2b, 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 severe constraints on the possible existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed in GRB 990123 is fainter than the optical echo expected in a dense red supergiant environment from an isotropic prompt 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.