Low-energy absorption towards the ultra-compact X-ray binary 4U 1850-087 located in the globular cluster NGC 6712

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica “G. Occhialini”, IASF/INAF, via Bassini 15, 20133 Milano, Italy e-mail: sidoli@iasf.mi.cnr.it
² Science Payload and Advanced Concepts Office, ESA, ESTEC, Postbus 299, 2200 AG, Noordwijk, The Netherlands
³ Research and Scientific Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands

Received: 16 May 2005
Accepted: 2 July 2005

Abstract

We report the results of two XMM-Newton observations of the ultra-compact low-mass X-ray binary 4U 1850-087 located in the galactic globular cluster NGC 6712. A broad emission feature at 0.7 keV was detected in an earlier ASCA observation and explained as the result of an unusual Ne/O abundance ratio in the absorbing material local to the source. We find no evidence for this feature and derive Ne/O ratios in the range 0.14–0.21, consistent with that of the interstellar medium. During the second observation, when the source was ~10% more luminous, there is some evidence for a slightly higher Ne/O ratio and additional absorption. Changes in the Ne/O abundance ratio have been detected from another ultra-compact binary, 4U 1543-624. We propose that these changes result from an X-ray–induced wind which is evaporated from an O and Ne rich degenerate donor. As the source X-ray intensity increases so does the amount of evaporation and hence the column densities and abundance ratio of Ne and O.