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A.N. Parmar (ESA), L. Boirin (SRON), D. Barret (CESR), S. Paltani (Lab d'Astrophysique de Marseille), J. Grindlay (CfA), N. Webb (CESR)
We report the discovery of narrow X-ray absorption lines from the dipping low-mass X-ray binary (LMXB) XB 1916-053 during an XMM-Newton observation in 2002 September. Two absorption lines were clearly detected at 6.65 and 6.95 keV in the EPIC pn persistent emission (non-dipping) spectrum, and identified with ka resonnant absorption from Fe xxv and Fe xxvi. In addition, there is marginal evidence for three other absorption features at 7.82 keV, 8.29 keV and 2.67 keV, consistent with Ni xxvii ka, Fe xxvi kb and S xvi transitions, respectively. The detection of these absorption lines from H-like and He-like ions indicate that a highly ionized plasma is present in the system. As both Fe xxv and Fe xxvi absorption lines are present, we can estimate the photo-ionization parameter, of the material responsible for these lines, to be 103.92. The Fe xxv absorption line is also detected during shallow, intermediate and deep dipping emission. From upper-limits on the EW of the Fe xxvi absorption line, we estimate log(xi) to be 3.83, 3.68 and 3.68, during shallow, intermediate and deep dipping, respectively. These values are consistent with a decrease amount of ionization as dipping activity increases. This is consistent with the presence of cooler material in the line of sight during dipping. We also report the discovery of an absorption edge at 0.98 keV in the persistent emission spectrum. The edge energy is observed to decrease to 0.87 keV as the source evolves towards deep dipping. This suggests that the detected edge represents a complex of edges from ions in a range of ionization states (likely moderately ionized Ne and/or Fe), and that the average ionization level decreases from persistent to deep dipping.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.