Discovery of a strongly phase-variable spectral feature in the isolated neutron star RX J0720.4-3125

Understand

We present the discovery of a strongly phase-variable absorption feature in the X-ray spectrum of the nearby, thermally-emitting, isolated neutron star RX J0720.4-3125.

• The absorption line was detected performing detailed phase-resolved spectroscopy in 20 XMM-Newton observations, covering the period May 2000 - September 2012.
• The feature has an energy of ~750eV, an equivalent width of ~30eV, and it is significantly detected for only ~20% of the pulsar rotation.
• The absorption feature appears to be stable over the timespan covered by the observations.