HST and VLT observations of the neutron star 1E 1207.4-5209

The peculiar central compact object 1E 1207.4-5209 in the G296.5+10.0 supernova remnant has been proposed to be an "anti-magnetar" - a young neutron star born with a weak dipole field. Accretion, possibly of supernova fallback material, has also been invoked to explain a large surface temperature anisotropy as well as the generation of peculiar cyclotron absorption features superimposed on its thermal spectrum. Interestingly enough, a faint optical/infrared source was proposed as a possible counterpart to 1E 1207.4-5209, but later questioned, based on coarse positional coincidence. On the basis of the large offset of 1E 1207.4-5209 with respect to the center of its host supernova remnant, the source should move at similar to 70 mas yr(-1). Thus, we tested the association by measuring the proper motion of the proposed optical counterpart. Using Hubble Space Telescope (HST) observations spanning 3.75 years, we computed a 3s upper limit of 7 mas yr(-1). Absolute astrometry on the same HST data set also places the optical source significantly off the 99% confidence Chandra position. This allows us to safely rule out the association. Using the HST data set, coupled to ground-based observations collected at the ESO/Very Large Telescope (VLT), we set the deepest limits ever obtained on the optical/infrared emission from 1E 1207.4-5209. By combining these limits with the constraints derived from X-ray timing, we rule out accretion as the source of the thermal anisotropy of the neutron star.