Evidence for a Binary Companion to the Central Compact Object 1E   1207.4-5209
  Unique among neutron stars, 1E 1207.4-5209 is an X-ray pulsar with a spin period of 424 ms that contains at least two strong absorption features in its energy spectrum. This neutron star has been identified as a member of the radio-quiet compact central objects in supernova remnants. It has been found that 1E 1207.4-5209 is not spinning down monotonically suggesting that this neutron star undergoes strong, frequent glitches, contains a fall-back disk, or possess a binary companion. Here, we report on a sequence of seven XMM-Newton observations of 1E 1207.4-5209 performed during a 40 day window in June/July 2005. Due to unanticipated variance in the phase measurements beyond the statistical uncertainties, we could not identify a unique phase-coherent timing solution. The three most probable timing solutions give frequency time derivatives of +0.9, -2.6, and +1.6 X 10^(-12) Hz/s (listed in descending order of significance). We conclude that the local frequency derivative during our XMM-Newton observing campaign differs from the long-term spin-down rate by more than an order of magnitude, effectively ruling out glitch models for 1E 1207.4-5209. If the long-term spin frequency variations are caused by timing noise, the strength of the timing noise in 1E 1207.4-5209 is much stronger than in other pulsars with similar period derivatives. Therefore, it is highly unlikely that the spin variations are caused by the same physical process that causes timing noise in other isolated pulsars. The most plausible scenario for the observed spin irregularities is the presence of a binary companion to 1E 1207.4-5209. We identified a family of orbital solutions that are consistent with our phase-connected timing solution, archival frequency measurements, and constraints on the companions mass imposed by deep IR and optical observations. 
Peter M. Woods (Dynetics/USRA/NSSTC), Vyacheslav E. Zavlin (MSFC) &   George G. Pavlov (Penn State Univ)