The X-ray spectrum of the bursting atoll source 4U 1728-34 observed with INTEGRAL

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France e-mail: mfalanga@cea.fr
² Unité mixte de recherche Astroparticule et Cosmologie, 11 place Berthelot, 75005 Paris, France
³ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano, via Bassini 15, 20133 Milano, Italy
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica cosmica, via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁶ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

Received: 28 March 2006
Accepted: 10 July 2006

Abstract

We present for the first time a study of the 3–200 keV broad band spectra of the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity diagram. The analysis was done using the INTEGRAL public and Galactic Center deep exposure data ranging from February 2003 to October 2004. The spectra are well described by a thermal Comptonization model with an electron temperature from 35 keV to 3 keV and Thomson optical depth, , from 0.5 to 5 in a slab geometry. The source undergoes a transition from an intermediate/hard to a soft state where the source luminosity increases from 2 to 12% of Eddington. We have also detected 36 type I X-ray bursts two of which show photospheric radius expansion. The energetic bursts with photospheric radius expansion occurred at an inferred low mass accretion rate per unit area of  g cm^-2 s^-1, while the others at a higher one between  g m^-2 s^-1. For 4U 1728-34 the bursts' total fluence, and the bursts' peak flux are anti-correlated with the mass accretion rate. The type I X-ray bursts involve pure helium burning either during the hard state, or during the soft state of the source.