Baryon Loading of Gamma-Ray Burst by Neutron Pickup

It is proposed that the baryons in gamma-ray burst (GRB) fireballs originate as "pickup" neutrons that leak in sideways from surrounding baryonic matter and convert to protons in a collision avalanche. The asymptotic Lorentz factor is estimated and, in the absence of collimation, is shown to be angle-dependent. Reasonable agreement is obtained with existing limits on the GRB baryonic component. The charged decay and collision products of the neutrons become ultrarelativistic immediately, and an ultra-high-energy neutrino burst is produced with an efficiency that can exceed 0.5. Other signatures may include lithium, beryllium, and/or boron lines in the supernova remnants associated with GRBs and the high polarization of the gamma rays.