A self-consistent approach to the hard and soft states of 4U 1705-44^*

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Italy e-mail: dai@fisica.unipa.it
² Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA
³ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
⁴ INAF - Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
⁵ IAAT, University of Tubingen, Sand 1, 72076 Tubingen, Germany
⁶ INAF IASF di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
⁷ Dipartimento di Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁸ Astronomical Institute, Academy of Sciences of the Czech Republic, Bocni II 1401a, 14131 Prague, Czech Republic

Received: 27 November 2009
Accepted: 25 February 2010

Abstract

Context. High-resolution spectroscopy has recently revealed in many low-mass X-ray binaries hosting a neutron star that the shape of the broad iron line observed in the 6.4–6.97 keV range is consistently well-fitted by a relativistically smeared line profile.

Aims. The presence of other broad features, besides the iron line, together with a high S/N of the spectra offer the possibility of testing a self-consistent approach to the overall broadband reflection spectrum and evaluating the impact of the reflection component in the formation of the broadband X-ray spectra.

Methods. We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a hard low flux state, the second during a soft, high-flux state. Both the spectra show broad iron emission lines. We fit the spectra using a two-component model, together with a reflection model specifically suited to the case of a neutron star, where the incident spectrum has a blackbody shape.

Results. In the soft state, the reflection model, convolved with a relativistic smearing component, consistently describes the broad features present in the spectrum, and we find a clear relation between the temperature of the incident flux and the temperature of the harder X-ray component that we interpret as the boundary layer emission. In this state we find converging evidence that the boundary layer outer radius is ~2 times the neutron star radius. In the low flux state, we observe a change in the continuum shape of the spectrum with respect to the soft state. Still, the broad local emission features can be associated with a disk reflecting matter, but in a lower ionization state, and possibly produced in an accretion disk truncated at greater distance.

Conclusions. Our analysis provides strong evidence that the reflection component in soft states of LMXBs comes from to hard X-ray thermal irradiation, which we identify with the boundary layer emission, also present in the continuum model. In the hard state, the broad iron line if also produced by reflection, and the continuum disk emission can be self-consistently accounted if the disk is truncated at a greater distance than the soft state.