Search for linear polarization in high energy gamma-ray sources

The azimuthal distribution of planes containing e⁺/e^- pairs from high-energy photon materialization is reminiscent, through a quadrupole anisotropy, of the degree and position angle of linear polarization of the incident photons. Data from the COS-B spark chamber are used in a search for such an effect in >50 MeV photons from bright sources, such as Vela, Crab, Geminga and a reference galactic plane region in Cygnus. Only for the Vela pulsar a low-chance-probability effect is found apparently implying a high (≡100%) degree of linear polarization for the Vela photons. This has important implications on the physics of the production mechanisms as well as of their geometry.