X-ray Characterization of Fermi Gamma-ray Pulsars

We assess the X-ray behavior of the Fermi-LAT gamma-ray pulsars. Although most of the rotational energy of gamma-ray pulsars is released in the GeV range, significant X-ray emission is often detected. Indeed, 67 gamma-ray pulsars have been seen in X-rays (30 radio-loud, 19 radio-quiet and 18 millisecond pulsars). Although the coverage is uneven, we try to characterize such X-ray emission, in the 0.3-10 keV range, in order to assess the relationship, if any, between the pulsars' behavior in X and gamma-rays for the three different populations of Fermi pulsars. While the most energetic pulsars do have low ratios of gamma-ray to non-thermal X-ray energy flux (Fgamma/Fx), little can be said for the bulk of Fermi neutron stars, since pulsars with similar energetics have Fgamma/Fx spanning three decades. While the least scattered sample is that of the millisecond pulsars, among the young neutron stars detected by Fermi, radio-quiet pulsars show, on average, higher values of Fgamma/Fx and they appear definitely less scattered than the radio-loud pulsars.