Geminga's Soft X-Ray Emission and the Structure of Its Surface

We present a model to explain the decrease in the amplitude of the pulse profile with increasing energy observed in Geminga's soft X-ray surface thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions emit spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface. The strongly pulsed emission from the plates dominates at low energy, while the surrounding's emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrative example, the plates are assumed to be magnetized, while the rest of the surface is field free. This plate structure may be seen as a schematic representation of a continuous but very nonuniform distribution of the surface magnetic field or as a quasi-realistic structure induced by past tectonic activity on Geminga.