The spectral variability of the gamma-ray emission from Geminga and VELA and its implications.

The gamma-ray emissions from Geminga and from the Interpeak 1 component of the Vela pulsar were analyzed in order to compare their spectral properties between 50 MeV and 5 GeV and to study their variability between 1975 and 1982 using the COS-B data. The stability of both sources above about 200 and 300 MeV, respectively, retaining an E exp -2.0 +/- 0.1 spectrum of constant intensity over these years, is remarkable. At lower energies, however, their spectral distribution has been found to be variable at the 4 sigma and 5 sigma level, respectively. Both sources exhibit a high state, characterized by a power-law spectrum of index close to -2 that extends down to 50 MeV. In a low state, their soft radiation below a few hundred MeV is largely suppressed for long periods. The induced spectral breaks in the low states were detected at 5 sigma and 4 sigma significance, respectively. The parallel behavior of the two sources hints at a common origin of their very hard gamma radiation and of its long term variability. It may find its roots in the properties of curvature radiation or very small pitch-angle synchrotron radiation, as discussed here.