2017/03/15 by A. Borghese, N. Rea, F. Coti Zelati +3 · 1 citation
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.1093/mnras/stx632
10 pages, 5 figures, 4 tables. Accepted for publication in MNRAS
arxiv created 2017/03/15 · arxiv updated 2017/05/03
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 June). 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 Bloop ∼ 1.7 × 1014 G, about a factor of ∼5 higher than the surface dipolar magnetic field (Bsurf ∼ 3.4 × 1013 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.