Narrow phase-dependent features in X-ray dim isolated neutron stars: a new detection and upper limits

Abstract

We report on the results of a detailed phase-resolved spectroscopy of archival XMM-Newton observations of X-ray Dim Isolated Neutron Stars (XDINSs). Our analysis revealed a narrow and phase-variable absorption feature in the X-ray spectrum of RX J1308.6+2127. The feature has an energy of ∼740 eV and an equivalent width of ∼15 eV. It is detected only in ∼ 1/5 of the phase cycle, and appears to be present for the entire timespan covered by the observations (2001 December -2007. The strong dependence on the pulsar rotation and the narrow width suggest that the feature is likely due to resonant cyclotron absorption/scattering in a confined high-B structure close to the stellar surface. Assuming a proton cyclotron line, the magnetic field strength in the loop is B loop ∼ 1.7 × 10 14 G, about a factor of ∼5 higher than the surface dipolar magnetic field (B surf ∼ 3.4 × 10 13 G). This feature is similar to that recently detected in another XDINS, RX J0720.4-3125, showing (as expected by theoretical simulations) that small scale magnetic loops close to the surface might be common to many highly magnetic neutron stars (although difficult to detect with current X-ray instruments). Furthermore, we investigated the available XMM-Newton data of all XDINSs in search for similar narrow phase-dependent features, but could derive only upper limits for all the other sources.

The outburst decay of the low magnetic field magnetar SWIFT J1822.3−1606: phase-resolved analysis and evidence for a variable cyclotron feature

“…There is also another feature at lower energy (∼ 2 keV) at phase ∼ 0.5, which, if significant, may be just a continuation of the main feature (this is supported by the line width evolution but not by the line energy evolution, see the two upper panels of Fig.11). A similar phase and energy dependent feature has been detected so far only in two sources: the low-B magnetar SGR 0418 (see T13) and the X-ray dim isolated neutron star RX J0720.4-3125 (see Borghese et al 2015). s in these sources the feature may be due to proton-cyclotron resonant scattering of X-ray photons emitted by the star surface onto charges flowing in a small coronal magnetic loop (for alternative interpretations see the discussion in T13 and Borghese et al 2015).…”

“…A similar phase and energy dependent feature has been detected so far only in two sources: the low-B magnetar SGR 0418 (see T13) and the X-ray dim isolated neutron star RX J0720.4-3125 (see Borghese et al 2015). s in these sources the feature may be due to proton-cyclotron resonant scattering of X-ray photons emitted by the star surface onto charges flowing in a small coronal magnetic loop (for alternative interpretations see the discussion in T13 and Borghese et al 2015). The energy variation of the line would be caused by magnetic field gradients along the coronal loop: as the neutron star rotates, photons directed toward us intercept sections of the loop with different magnetic field intensities.…”

Cooling theory faced with old warm neutron stars: role of non-equilibrium processes with proton and neutron gaps

“…This discrepancy may be due to large magnetic fields of these XDINSs. In fact, XDINSs commonly have absorption feature in the X-ray spectrum [102,103], which is interpreted as the proton cyclotron resonance or atomic transition. Both require the magnetic field larger than about 10 13 G, and such a strong magnetic field can affect the thermal evolution of XDINSs.…”

Strongly Magnetized Pulsars: Explosive Events and Evolution

“…In particular, the magneto-thermal evolutionary tracks computed by Viganò et al (2013) show that their observational characteristics are compatible with those expected from neutron stars born with dipolar field of a few 10 14 G after a few millions of years, a timespan consistent with the characteristic ages of XDINSs. Borghese et al (2015Borghese et al ( , 2017 inspected the phase resolved spectra of all XDINSs and found in two of them narrow phase-dependent absorption features that, if interpreted as proton cyclotron lines, akin in SGR 0418+5729, indicate the presence close to the star surface of nondipolar magnetic field components ≈10 times stronger than the values derived from the timing parameters, tightening the links between XDINSs and magnetars.…”