Is RX J0720.4-3125 a freely precessing neutron starquestionMark

RX J0720.43125 belongs to a group of radioquiet isolated neutron stars, whoseXray spectra consist of broad absorption feature(s) superimposed on a blackbodycontinuum. It is unique in that its spectrum and flux are time variable. Thecause for this is unclear, but one possibility is free precession of twodifferent hot spots at the star surface. Free precession can reveal importantinformation on the star.s interior structure. However, the last observationshows that the spectral evolution does not follow the prediction of a simplesinusoidal model. Continued monitoring of the spectral and timing properties isneeded to reveal whether this is due to a complex and patchy surface thermal mapor whether the variation is caused by effects different from precession. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]