Compton Echoes from Gamma‐Ray Bursts

Recent observations of gamma-ray bursts (GRBs) have provided growing evidencefor collimated outflows and emission, and strengthened the connection betweenGRBs and supernovae. If massive stars are the progenitors of GRBs, the hardphoton pulse will propagate in the pre-burst, dense environment. Circumstellarmaterial will Compton scatter the prompt GRB radiation and give rise to areflection echo. We calculate luminosities, spectra, and light curves of suchCompton echoes in a variety of emission geometries and ambient gasdistributions, and show that the delayed hard X-ray flash from a pulsepropagating into a red supergiant wind could be detectable by Swift out toz~0.2. Independently of the gamma-ray spectrum of the prompt burst, reflectionechoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^2because of Compton downscattering. At fixed burst energy per steradian, theluminosity of the reflected echo is proportional to the beaming solid angle,Omega_b, of the prompt pulse, while the number of bright echoes detectable inthe sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it issmaller in the case of more collimated jets. The lack of an X-ray echo at onemonth delay from the explosion poses severe constraints on the possibleexistence of a lateral GRB jet in SN 1987A. The late r-band afterglow observedin GRB990123 is fainter than the optical echo expected in a dense redsupergiant environment from a isotropic prompt optical flash. Significant MeVdelayed emission may be produced through the bulk Compton (or Compton drag)effect resulting from the interaction of the decelerating fireball with thescattered X-ray radiation.Comment: LaTeX, 18 pages, 4 figures, revised version accepted for publication in the Ap