X-ray spectroscopy of MXB 1728–34 with XMM-Newton

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
e-mail: elise.egron@dsf.unica.it
² Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
³ Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
⁴ INAF – Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy

Received: 6 November 2010
Accepted: 29 March 2011

Abstract

We analyzed an XMM-Newton observation of the low-mass X-ray binary and atoll source MXB 1728–34. The source was in a low-luminosity state during the XMM-Newton observation, corresponding to a bolometric X-ray luminosity of $\mathrm{5}\mathrm{\times }{\mathrm{10}}^{\mathrm{36}}{\mathit{d}}_{\mathrm{5.1}\mathrm{kpc}}^{\mathrm{2}}$ erg s^-1. The 1–11 keV X-ray spectrum of the source, obtained combining data from all the five instruments on-board XMM-Newton, is well fitted by a Comptonized continuum. Evident residuals are present at 6−7 keV, which are ascribed to the presence of a broad iron emission line. This feature can be equally well fitted by a relativistically smeared line or by a self-consistent, relativistically smeared reflection model. Under the hypothesis that the iron line is produced by reflection from the inner accretion disk, we can infer important information on the physical parameters of the system, such as the inner disk radius, R_in = 25−100 km, and the inclination of the system, 44° < i < 60°.