Narrow absorption features in the co-added XMM-Newton RGS spectra of isolated neutron stars

We co-added the available XMM-Newton Reflection Grating Spectrometer (RGS) spectra for each of the isolated X-ray pulsars RX J0720.4−3125, RX J1308.6+2127 (RBS1223), RX J1605.3+3249 and RX J1856.4−3754 (four members of the ‘Magnificent Seven’) and the ‘Three Musketeers’ Geminga, PSR B0656+14 and PSR B1055−52. We confirm the detection of a narrow absorption feature at 0.57 keV in the co-added RGS spectra of RX J0720.4−3125 and RX J1605.3+3249 (including most recent observations). In addition, we found similar absorption features in the spectra of RX J1308.6+2127 (at 0.53 keV) and maybe PSR B1055−52 (at 0.56 keV). The absorption feature in the spectra of RX J1308.6+2127 is broader than the feature e.g. in RX J0720.4−3125. The narrow absorption features are detected with 2-5.6σ significance.

Although very bright and frequently observed, there are no absorption features visible in the spectra of RX J1856.4−3754 and PSR B0656+14, while the co-added XMM-Newton RGS spectrum of Geminga has not enough counts to detect such a feature.

We discuss a possible origin of these absorption features as lines caused by the presence of highly ionized oxygen (in particular O vii and/or O vi at 0.57 keV) in the interstellar medium and absorption in the neutron star atmosphere, namely the absorption features at 0.57 keV as gravitational redshifted (gr= 1.17) O viii.