Discovery of a redshifted X-ray emission line in the symbiotic neutron star binary 4U 1700+24

Abstract

We present the spectral analysis of an XMM-Newton observation of the X-ray binary 4U 1700+24, performed during an outburst in August 2002. The EPIC-PN spectrum above 1 keV can be modeled by a blackbody plus Comptonization model, as in previous observations. At lower energies, however, we detect a prominent soft excess, which we model with a broad Gaussian centered at ∼0.5 keV. In the high resolution RGS spectrum we detect a single emission line, centered at 19.19-0.09+0.05 Å. We discuss two possible interpretations for this line: O VIII at redshift z = 0.012 -0.004+0.002 or Ne IX at redshift z ∼ 0.4. © ESO 2005.