X‐Ray observations of “γ‐ray only” pulsars Available to Purchase

Fermi‐LAT and AGILE have already detected more than 70 rotation‐powered pulsars at GeV energies, opening a new era of pulsar physics. In particular, Fermi has unveiled the existence of a large population of Geminga‐like pulsars, γ‐ray bright but radio‐silent. We used XMM‐Newton, Chandra and Swift to study such new, γ‐ray selected, pulsar population and to compare its X‐ray behaviour with that of the radio pulsars. While radio‐loud and radio‐quiet pulsars need not to be different objects, their different viewing geometry with respect to the observer does influence the ratio between γ and X‐ray emissions. When plotting the distance‐indipendent γ to X‐ray flux ratios as a function of the pulsars’ rotational energy losses, one immediately sees that pulsars with similar energetics have $Fγ/FX$ spanning 3 decades. Such spread, most probably stemming from vastly different geometrical configurations of the X and γ‐ray emitting regions, defies any straightforward interpretation of the plot. We find that, on average, radio‐quiet pulsars do have higher values of $Fγ/FX,$ implying an intrinsec faintness of their X‐ray emission and/or a different geometrical configuration.