1E1207.4-5209 - a Unique Object
  The discovery of deep spectral features in the X-ray spectrum of 1E1207.4-5209 focussed the attention of the astronomical community on this radio-quiet NS, making it the most intensively observed INS ever. The harvest of X-ray photons, collected mainly by XMM-Newton, unveiled a number of unique characteristics, raising questions on this source very nature. 
G.F. Bignami, A. De Luca, P.A. Caraveo, S. Mereghetti, M. Moroni, R.P.   Mignani