Spectral and rotational changes in the isolated neutron star RX J0720.4-3125

RX J0720.4-3125 is an isolated neutron star that, uniquely in its class, has shown changes in its thermal X-ray spectrum. We use new spectra taken with Chandra's Low Energy Transmission Grating Spectrometer, as well as archival observations, to try to understand the timescale and nature of these changes. We construct light curves, which show both small, slow variations on a timescale of years and a larger event that occurred more quickly, within half a year. From timing, we find evidence for a "glitch" coincident with this larger event, with a fractional increase in spin frequency of ∼5 × 10 ^-8. We compare the "before" and "after" spectra with those from RX J1308.6+2127, an isolated neutron star with similar temperature and magnetic field strength, but with a much stronger absorption feature in its spectrum. We find that the "after" spectrum can be represented remarkably well by the superposition of the "before" spectrum, scaled by two-thirds, and the spectrum of RX J1308.6+2127, thus suggesting that the event affected approximately one-third of the surface. We speculate that the event reflects a change in surface composition caused by, e.g., an accretion episode.