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

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano “G.Occhialini” – INAF, via Bassini 15, Milano 20133, Italy e-mail: tiengo@mi.iasf.cnr.it
² Università degli Studi di Milano, Dipartimento di Fisica, via Celoria 16, 20133 Milano, Italy
³ Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
⁴ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁵ SRON, National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ NSF Astronomy and Astrophysics Fellow
⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 25 November 2003
Accepted: 22 June 2005

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 Å. We discuss two possible interpretations for this line: O VIII at redshift or Ne IX at redshift .