Monitoring the spectral variability of the isolated neutron star RX J0720.4-3125

RX J0720.43125 belongs to a small group radioquiet isolated neutron stars,whose Xray spectra consist of a broad absorption feature superimposed on ablackbody continuum. However, it is unique in that it is the only source inwhich time evolution of the absorption feature and thermal component has beenobserved. The origin of the changes is still unclear, but their study is ofgreat potential importance for revealing physical properties of isolated neutronstars. The spectral evolution may be caused by changes in the star.s viewingangle, thermal and/or magnetic field distribution. To distinguish between thesepossibilities, we propose to continue monitoring RX J0720.43125. This will alsohelp to further constrain the timing properties of this intriguing source. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]