Phase-dependent absorption features in X-ray spectra of X-ray Dim Isolated Neutron Stars

A detailed phase-resolved spectroscopy of archival XMM–Newtonobservations of X-ray Dim Isolated Neutron Stars (XDINSs) led to thediscovery of narrow and strongly phase-dependent absorption features intwo of these sources. The first was discovered in the X-ray spectrum ofRX J0720.4-3125, followed by a new possible candidate in RXJ1308.6+2127. Both spectral lines have similar properties: they aredetected for only ∼ 20% of the rotational cycle and appear to bestable over the timespan covered by the observations. We performed MonteCarlo simulations to test the significance of these phase-variablefeatures and in both cases the outcome has confirmed the detection witha confidence level > 4.6σ. Because of the narrow width and thestrong dependence on the pulsar rotational phase, the most likelyinterpretation for these spectral features is in terms of resonantproton cyclotron absorption scattering in a confined high-B structureclose to the stellar surface. Within the framework of thisinterpretation, our results provide evidence for deviations from a puredipole magnetic field on small scales for highly magnetized neutronstars and support the proposed scenario of XDINSs being aged magnetars,with a strong non-dipolar crustal B-field component.