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

📝 Abstract

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. 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. 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 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 (be it a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behaviour of the hard tail: high values of accretion rates correspond to an efficient Bulk Comptonization process (bulk parameter delta > 0) while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (delta=0).

💡 Analysis

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. 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. 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 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 (be it a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behaviour of the hard tail: high values of accretion rates correspond to an efficient Bulk Comptonization process (bulk parameter delta > 0) while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (delta=0).

📄 Content

arXiv:0912.2707v1 [astro-ph.HE] 14 Dec 2009 Astronomy & Astrophysics manuscript no. draft c⃝ESO 2018 October 31, 2018 Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model L.I. Mainardi1, A. Paizis1, R.Farinelli2, E. Kuulkers3, J. Rodriguez4, D. Hannikainen5, P. Savolainen5, S. Piraino6,7, A. Bazzano8, A. Santangelo7 1 INAF-IASF, Sezione di Milano, Via Bassini 15, I–20133 Milano, Italy 2 Dipartimento di Fisica, Universit`a di Ferrara, Via Saragat 1,I–44100 Ferrara, Italy 3 ESAC, ISOC, Villa˜nueva de la Ca˜nada, Madrid, Spain 4 CNRS, FRE 2591, CE Saclay DSM/DAPNIA/SAp, F–91191 Gif sur Yvette Cedex, France 5 Mets¨ahovi Radio Observatory, TKK, Mets¨ahovintite 114, FI-02540 Kylm¨al¨a, Finland 6 INAF-IASF, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy 7 IAAT, University of T¨ubingen, Sand 1, 72076 T¨ubingen, Germany 8 INAF-IASF, Sezione di Roma, Via del Fosso del Cavaliere 100, I-00133, Roma, Italy Received / Accepted 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 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, kTs >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 (be it a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behaviour of the hard tail: high values of accretion rates correspond to an efficient Bulk Comptonization process (bulk parameter δ ̸= 0) 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. Key words. Stars: individual: GX 5–1, GX 13+1, GX 3+1, GX 9+1, GX 349+2 – X-rays: binaries – binaries: close – stars: neutron – accretion

1. Introduction Low-Mass X-ray Binaries (LMXBs) are systems where a com- pact object, either a neutron star (NS) or a black hole candi- date (BHC), accretes matter via Roche lobe overflow from a normal companion of mass M ≲1M⊙; a peculiar characteris- tic to this type of system is the formation of an accretion disk in the orbital plane near the compact object. In this paper we studied persistently bright NS LMXBs (LX ≈1037 −1038 erg sec−1) granting long-lasting observability with instruments operating in the soft/hard X-ray range. The spectra of these sources are usually described as the sum of two components Send offprint requests to: A. Paizis, ada@iasf-milano.inaf.it (e.g Mitsuda et al. 1984; White et al. 1988; Barret 2001): a soft component often likely associated with the accretion disk or NS and a hard component interpreted as thermal Comptonization of soft seed photons from the disk and/or the NS by high temperature plasma of electrons near the compact object (so- called corona). The advent of broad-band X-ray missions, such as BeppoSAX, RXTE, INTEGRAL, revealed the presence of a spectral hardening (so-called “hard tails”) above ∼30 keV on top of otherwise soft spectra (e.g Di Salvo & Stella 2002; Paizis et al. 2006, hereafter P06, and references therein). These hard tails, mostly fit with phenomenological models such as a power-law, have been detected in Z sources (e.g. Frontera et al. 2 Mainardi et al.: Spectral evolution of bright NS LMXBs with INTEGRAL 1998; Di Salvo et al. 2002) and also in the bright Atoll source GX 13+1 (P06). To explain the origin of these hard tails, different models have been proposed across the years, such as direct synchrotron emission from a jet (Markoff et al. 2005), hybrid thermal/non- thermal Comptonisation (Coppi 1999; Di Salvo et al. 2006) or, more recently, bulk motion Comptonisation (Titarchuk et al. 1997; Farinelli et al. 2008, hereafter TMK97 and F08, respec- tively). The adoption of different mo

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