Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model

¹ INAF-IASF, Sezione di Milano, via Bassini 15, 20133 Milano, Italy e-mail: ada@iasf-milano.inaf.it
² Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
³ ESAC, ISOC, Villañueva de la Cañada, Madrid, Spain
⁴ CNRS, FRE 2591, CE Saclay DSM/DAPNIA/SAp, 91191 Gif sur Yvette Cedex, France
⁵ Metsähovi Radio Observatory, TKK, Metsähovintite 114, 02540 Kylmälä, Finland
⁶ INAF-IASF, Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
⁷ IAAT, University of Tübingen, Sand 1, 72076 Tübingen, Germany
⁸ INAF-IASF, Sezione di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 17 July 2009
Accepted: 9 December 2009

Abstract

Aims. The aim of this work is to investigate in a physical and quantitative way the spectral evolution of bright neutron star low-mass X-ray binaries (NS LMXBs) with special regard to the transient hard X-ray tails.

Methods. We analyzed INTEGRAL data for five sources (GX 5–1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source, X-ray spectra from different states were fitted with the recently proposed model compTB.

Results. The spectra have been fit with a two-compTB model. In all cases the first compTB describes the dominant part of the spectrum that we interpret as thermal Comptonization of soft seed photons (<1 keV), likely from the accretion disk, by a 3-5 keV corona. In all cases, this component does not evolve much in terms of Comptonization efficiency, with the system converging to thermal equilibrium for an increasing accretion rate. The second compTB varies more dramatically, spanning from bulk plus thermal Comptonization of blackbody seed photons to the blackbody emission alone. These seed photons (R < 12 km, kT_s > 1 keV), likely from the neutron star and the innermost part of the system, the transition layer, are Comptonized by matter in a converging flow. The presence and nature of this second compTB component (whether a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behavior of the hard tail: high values of accretion rates correspond to an efficient bulk Comptonization process (bulk parameter ), while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (δ = 0).

Conclusions. The spectral evolution of the sources has been successfully studied in terms of thermal and bulk Comptonization efficiency in relation to the physical conditions in the transition layer.