Differences between radio-loud and radio-quiet gamma-ray pulsars as revealed by Fermi

By comparing the properties of non-recycled radio-loud (RL) $\gamma$-ray pulsars and radio-quiet (RQ) $\gamma$-ray pulsars, we have identified the differences between these two populations. We found that the $\gamma$-ray spectral curvatures of RQ pulsars are significantly larger than those of RL pulsars. The distributions of the magnetic field strength at the light cylinder $B_{\rm LC}$ are also found to be different in these two populations. We have also confirmed the previously reported difference of $\gamma$-ray to X-ray flux ratios, though the significance can be hampered by their statistical uncertainties. While positive correlation between $B_{\rm LC}$ and the cutoff energies has been found in the RQ pulsars, such relation is absence in the RL population. In the context of outer gap model, we discuss the expected properties of these two populations and compare with the differences identified in our analysis.