Compton echoes from gamma-ray bursts: Unveiling misaligned jets in nearby type Ib/c supernovae

There is now compelling evidence of a link between long-duration gamma-ray bursts (GRBs) and Type Ib/c supernovae (SNe). These core-collapse explosions are conjectured to radiate an anisotropic, beamed component associated with a decelerating, relativistic outflow and an unbeamed, isotropic component associated with the slowly expanding stellar debris. The anisotropic emission remains at a very low level until the Doppler cone of the beam intersects the observer's line of sight, making off-axis GRB jets directly detectable only at long wavelengths and late times (≳1 yr). Circumstellar material, however, will Compton scatter the prompt gamma-ray and afterglow radiation flux and give rise to a reflection echo. We show that the Compton echo of a misaligned GRB carries an X-ray luminosity that may exceed by many orders of magnitude that produced by the underlying subrelativistic SN during the first few weeks. Bright scattering echoes may therefore provide a means for detecting a population of misaligned GRBs associated with nearby Type Ib/c SNe and yield crucial information on the environment surrounding a massive star at the time of its death. The question of whether or not the interpretation of GRB 980425 as an ordinary GRB observed off-axis is consistent with the lack of an X-ray echo is addressed, along with the constraints derived on the possible existence of misaligned GRB jets in SN 1993J, SN 1994I, SN 1999em, and SN 2002ap.