  Recent observations of gamma-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 pre-burst, 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 gamma-ray spectrum of the prompt burst, reflection
echoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^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 bright echoes detectable in
the sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it is
smaller in the case of more collimated jets. The lack of an X-ray echo at one
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 GRB990123 is fainter than the optical echo expected in a dense red
supergiant environment from a 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.
