TY - JOUR ID - discovery1386976 Y1 - 2012/03// UR - http://dx.doi.org/10.1111/j.1745-3933.2011.01209.x TI - A time-variable, phase-dependent emission line in the X-ray spectrum of the isolated neutron star RX J0822-4300 N1 - © 2012 RAS JF - Monthly Notices of the Royal Astronomical Society: Letters VL - 421, PB - WILEY A1 - De Luca, A A1 - Salvetti, D A1 - Sartori, A A1 - Esposito, P A1 - Tiengo, A A1 - Zane, S A1 - Turolla, R A1 - Pizzolato, F A1 - Mignani, RP A1 - Caraveo, P A1 - Mereghetti, S A1 - Bignami, GF N2 - RX J0822-4300 is the central compact object associated with the Puppis A supernova remnant. Previous X-ray observations suggested RX J0822-4300 to be a young neutron star with a weak dipole field and a peculiar surface temperature distribution dominated by two antipodal spots with different temperatures and sizes. An emission line at 0.8 keV was also detected. We performed a very deep (130-ks) observation with XMM-Newton, which allowed us to study in detail the phase-resolved properties of RX J0822-4300. Our new data confirm the existence of a narrow spectral feature, best modelled as an emission line, only seen in the 'soft'-phase interval - when the cooler region is best aligned to the line of sight. Surprisingly, comparison of our recent observations to the older ones yields evidence for a variation in the emission-line component, which can be modelled as a decrease in the central energy from ?0.80 keV in 2001 to ?0.73 keV in 2009-10. The line could be generated via cyclotron scattering of thermal photons in an optically-thin layer of gas, or, alternatively, it could originate in low-rate accretion by a debris disc. In any case, a variation in energy, pointing to a variation of the magnetic field in the line-emitting region, cannot be easily accounted for. SP - L72 EP - L76 IS - 1 AV - public ER -