Narrow absorption features in the co-addedXMM-NewtonRGS spectra of isolated neutron stars

ABSTRACT We co-added the availableXMM-Newton Reflection Grating Spectrometer (RGS) spectra for each of the isolated X-ray pulsars RXJ0720.4-3125, RXJ1308.6+2127 (RBS1223), RXJ1605.3+3249 and RXJ1856.4-3754 (four members of the 'Magnificent Seven') and the 'Three Musketeers' Geminga, PSRB0656+14 and PSRB1055-52. We confirm the detection of a narrow absorption feature at 0.57keV in the co-added RGS spectra of RXJ0720.4-3125 and RXJ1605.3+3249 (including most recent observations). In addition, we found similar absorption features in the spectra of RXJ1308.6+2127 (at 0.53keV) and maybe PSRB1055-52 (at 0.56keV). The absorption feature in the spectra of RXJ1308.6+2127 is broader than the feature e.g. in RXJ0720.4-3125. 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 RXJ1856.4-3754 and PSRB0656+14, while the co-addedXMM-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 Ovii and/or Ovi at 0.57keV) in the interstellar medium and absorption in the neutron star atmosphere, namely the absorption features at 0.57keV as gravitational redshifted (g r= 1.17) Oviii. [PUBLICATION ABSTRACT]