The iron K-shell features of MXB 1728–34 from a simultaneous Chandra-RXTE observation

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy e-mail: dai@fisica.unipa.it
² National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁴ Center for Space Research and Department of Physics, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139, USA
⁵ Astronomical Institute “Anton Pannekoek”, University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 12 April 2005
Accepted: 28 October 2005

Abstract

We report on a simultaneous Chandra and RossiXTE observation of the low-mass X-ray binary atoll bursting source MXB 1728–34 performed on 2002 March 3–5. We fit the 1.2–35 keV continuum spectrum with a blackbody plus a Comptonized component. Large residuals at 6–10 keV can be fitted by a broad ( keV) Gaussian emission line or, alternatively, by two absorption edges associated with lowly ionized iron and Fe XXV/XXVI at ~7.1 keV and ~9 keV, respectively. In this interpretation, we find no evidence of broad emission lines between 6 and 7 keV. We test our alternative model of the iron K shell region by reanalysing a previous BeppoSAX observation of MXB 1728–34, finding a general agreement with our new spectral model.