Exploring the statistical properties of gamma-ray normal pulsars

To study the normal pulsars based on the observations in radio and γ -ray bands, we try to investigate their classifications by the samples with the three different emission types, i.e., radio-only, γ -only and γ + radio emissions, and then discuss their physical causes. In the spin-down power range of Ė∼ 10^36-10^39 erg·s^-1 , three types of normal pulsars exhibit the similar distributions of the spin period– P , spin-down power– Ė and characteristic age– τ , implying that they may belong to the same category of young pulsars. Moreover, in the range of Ė∼ 10^33-10^36 erg·s^-1 , γ -only/ γ + radio normal pulsars share the similar distributions of P and τ values, and these two group sources exhibit the faster average spin periods ( ⟨ P⟩∼ 0.23 s/ ⟨ P⟩∼ 0.22 s), higher average spin-down powers ( ⟨Ė⟩∼ 2.00 × 10^35 erg·s^-1 / ⟨Ė⟩∼ 2.13 × 10^35 erg·s^-1 ) and smaller average characteristic ages ( ⟨τ⟩∼ 4.8× 10^5 yr/ ⟨τ⟩∼ 4.1× 10^5 yr) than radio-only normal pulsars ( ⟨ P⟩∼ 0.42 s, ⟨Ė⟩∼ 3.2× 10^34 erg·s^-1 and ⟨τ⟩∼ 1.64× 10^6 yr). We suggest that γ -only/ γ + radio normal pulsars with Ė∼ 10^33-10^36 erg·s^-1 are the young objects emitting γ -rays due to the high voltage in the radiation region. Meanwhile, after the temporal evolution of about τ∼ 10^6 yr, these two group pulsars can be hardly detected with γ -rays due to the decrease of the radiation voltage, which will finally evolve into radio-only pulsars.