The Surprising High-Energy Pulsations of PSR J1813-1246

The launch of Fermi has revolutionized our understanding of high-energy emission from pulsar magnetospheres. The comparison between radio and gamma-ray light curves has provided important information and constraints on emission geometry. On the other hand, only a few pulsars have long, dedicated observations in the X-ray band. We have obtained new, deep XMM-Newton and Chandra observations of the energetic, radio-quiet PSR J1813-1246 discovered in gamma rays by Fermi. Extending the Fermi ephemeris to five years, we found two glitches. The detected X-ray pulsations of J1813-1246 have a nearly100% pulsed fraction, with very sharp, non-thermal peaks separated by 0.5 in phase. The X-ray spectrum is unusual and very hard, with photon index of 0.85. The Fermi gamma-ray light curve also shows two peaks with 0.5 phase separation, that lag the X-ray peaks by 0.25 in phase. The unusual X-ray and gamma-ray phasing cannot be easily explained by current outer magnetosphere emission models. We explore an alternative geometrical model where the gamma-ray emission is from the outer magnetosphere while the X-ray emission comes from the polar cap pair cascades.