Astrophysical Limits on Lorentz Violation for All Charged Species
  If Lorentz violation exists, it will affect the thresholds for pair creation processes. Lorentz-violating operators that change the maximum velocities of charged particles may increase or decrease the extinction rate of gamma-rays moving through space. If the emissions from high-energy astrophysical sources do not show any signs of anomalous absorption, this allows us to place bounds on the Lorentz-violating c coefficients for multiple species of charged particles. The bounds for a species of mass m_X based on observing photons at an energy E can be O(m_X^2/E^2), which corresponds to limits at the 10^(-15)(m_X^2/m_e^2) level for the most energetic photons. 
Brett Altschul