Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209

The X-ray source 1E 1207.4-5209 is a compact central object in the G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with Chandra, suggests that it is a neutron star. The X-ray spectrum of this radio-quiet pulsar shows at least two absorption lines, the first spectral features discovered in radiation from an isolated neutron star. Here we report the results of timing analyses of Chandra and XMM-Newton observations of this source showing a nonmonotonous behavior of its period. We discuss three hypotheses that might explain the observational result. The first assumes that 1E 1207.4-5209 is a glitching pulsar, with frequency jumps of Δf ≳ 5 μHz occurring every 1-2 yr. The second hypothesis explains the deviations from a steady spin-down as being due to accretion (with the accretion rate varying from ∼10¹³ to ≳10¹⁶ g s^-1) from a disk possibly formed from ejecta produced in the supernova explosion. Finally, the period variations might be explained by assuming the pulsar is in a wide binary system with a long period P_orb ∼ 0.2-6 yr and a low-mass companion M₂ < 0.3 M_⊙.