HST and VLT observations of the neutron star 1E 1207.4–5209^⋆

¹ Istituto Universitario di Studi Superiori, Viale Lungo Ticino Sforza 56, 27100 Pavia, Italy
e-mail: deluca@lambrate.inaf.it
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via Bassini 15, 20133 Milano, Italy
³ Istituto Nazionale di Fisica Nucleare, Sez. di Pavia, via Bassi 6, 27100 Pavia, Italy
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁵ Università degli Studi di Pavia, Dipartimento di Fisica Nucleare e Teorica, via Bassi 6, 27100 Pavia, Italy
⁶ European Southern Observatory, Karl Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 12 May 2010
Accepted: 20 June 2010

Abstract

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  ~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 3σ 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.