XMM-Newton observations of the low-mass X-ray binary XB 1832–330  in the galactic globular cluster NGC 6652^*

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via E. Bassini 15, 20133 Milano, Italy e-mail: sidoli@iasf-milano.inaf.it
² Science Payload and Advanced Concepts Office, ESA, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
³ Research and Scientific Support Department of ESA, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands

Received: 4 February 2008
Accepted: 25 May 2008

Abstract

Context. We report on two XMM-Newton observations performed in 2006 of the luminous low-mass X-ray binary XB 1832–330, which is located in the galactic globular cluster NGC 6652 and is probably an ultracompact binary (orbital period, P_orb, of <1 h).

Aims. The aim of these observations is to investigate the low-energy absorption towards XB 1832–330 and in particular to search for the Ne-rich material local to the binary, which has been suggested as a possible spectral signature of Neon-rich, degenerate companions.

Methods. XMM-Newton observed the source twice, in 2006 September and October. High resolution spectroscopy with the XMM-Newton RGS was used to estimate the ratio of the neutral neon-to-oxygen column densities to search for an anomalous Ne abundance in this X-ray binary.

Results. We find no evidence of anomalous Ne/O ratios, finding Ne/O =  and Ne/O =  (1σ uncertainties) in the two observations. These values are consistent with the one in the interstellar medium. Timing analysis of EPIC data suggests possible periodicities at  s and  s, which need confirmation. A by-product of these observations consists of the spatial analysis of the source field, which resulted in the detection of 46 faint sources within the EPIC field of view, which are not present in the Second XMM-Newton Serendipitous Source Catalogue. All these faint sources are probably foreground objects.

Conclusions. We performed the first high spectral resolution observations of XB 1832–330, a probable ultracompact binary, without finding any evidence of an anomalous Ne abundance.