Abstract
A detailed phase-resolved spectroscopy of archival XMM–Newton observations of X-ray Dim Isolated Neutron Stars (XDINSs) led to the discovery of narrow and strongly phase-dependent absorption features in two of these sources. The first was discovered in the X-ray spectrum of RX J0720.4-3125, followed by a new possible candidate in RX J1308.6+2127. Both spectral lines have similar properties: they are detected for only ∼ 20% of the rotational cycle and appear to be stable over the timespan covered by the observations. We performed Monte Carlo simulations to test the significance of these phase-variable features and in both cases the outcome has confirmed the detection with a confidence level > 4.6σ. Because of the narrow width and the strong dependence on the pulsar rotational phase, the most likely interpretation for these spectral features is in terms of resonant proton cyclotron absorption scattering in a confined high-B structure close to the stellar surface. Within the framework of this interpretation, our results provide evidence for deviations from a pure dipole magnetic field on small scales for highly magnetized neutron stars and support the proposed scenario of XDINSs being aged magnetars, with a strong non-dipolar crustal B-field component.
Highlights
The X-ray Dim Isolated Neutron Stars (XDINSs) form a group of seven thermally-emitting, radio-quiet [1], nearby ( 500 pc) X-ray pulsars, originally discovered by ROSAT
XDINSs appear to be thermally cooling with emission in the soft X-ray band and faint optical and/or ultraviolet counterparts [4]
XDINSs show thermal spectra which can be well described by blackbody models with inferred temperature in the range kT ∼ 50 – 100 eV
Summary
The XDINSs form a group of seven thermally-emitting, radio-quiet [1], nearby ( 500 pc) X-ray pulsars, originally discovered by ROSAT (see [2] for a review). According to the most recent magneto-thermal evolution models [9], XDINSs are thought to be descendants of magnetars, we expect to find such phase-dependent features in their X-ray spectra. Left-hand, panel b: 0.6 – 0.8 phase-resolved spectrum fitted with an absorbed blackbody plus a Gaussian profile (solid line).
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