%0 Journal Article %A De Luca, A %A Salvetti, D %A Sartori, A %A Esposito, P %A Tiengo, A %A Zane, S %A Turolla, R %A Pizzolato, F %A Mignani, RP %A Caraveo, P %A Mereghetti, S %A Bignami, GF %D 2012 %F discovery:1386976 %I WILEY %J Monthly Notices of the Royal Astronomical Society: Letters %N 1 %P L72 - L76 %T A time-variable, phase-dependent emission line in the X-ray spectrum of the isolated neutron star RX J0822-4300 %U https://discovery.ucl.ac.uk/id/eprint/1386976/ %V 421, %X 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. %Z © 2012 RAS