Suzaku broad-band spectrum of 4U 1705-44: probing the reflection component in the hard state
Date Issued
2015
Author(s)
Di Salvo, T.
•
Iaria, R.
•
Matranga, M.
•
Burderi, L.
•
•
•
•
Riggio, A.
•
Robba, N. R.
•
Ueda, Y.
Description
We thank the unknown referee for useful comments which helped to improve the quality of the paper. The High-Energy Astrophysics Group of Palermo acknowledges support from the Fondo Finalizzato alla Ricerca (FFR) 2012/13, project no. 2012-ATE-0390, founded by the University of Palermo. This work was partially supported by the Regione Autonoma della Sardegna through POR-FSE Sardegna 2007-2013, L.R. 7/2007, Progetti di Ricerca di Base e Orientata, project no. CRP-60529, and by the INAF/PRIN 2012-6. EE acknowledges financial support from the Regione Autonoma della Sardegna through a research grant under the programme CRP-25399 PO Sardegna FSE 2007-2013, L.R. 7/2007. AP is supported by a Juan de la Cierva fellowship and acknowledges grants AYA2012-39303, SGR2009-811, and iLINK2011-0303.
Abstract
Iron emission lines at 6.4-6.97 keV, identified with Kα radiative transitions, are among the strongest discrete features in the X-ray band. These are one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent Suzaku observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705-44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disc. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM-Newton observation performed in the soft state. In particular, we find that the inner disc radius is Rin = 17 ± 5Rg (where Rg is the gravitational radius, GM/c2), the emissivity dependence from the disc radius is r-2.5 ± 0.5, the inclination angle with respect to the line of sight is i = 43° ± 5°, and the outer radius of the emitting region in the disc is Rout > 200Rg. We note that the accretion disc does not appear to be truncated at large radii, although the source is in a hard state at ̃3 per cent of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 ± 0.03 keV, compatible with emission from mildly ionized argon (Ar XVI-XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low-energy line appears compatible with being smeared by the same inner disc parameters found for the reflection component.
Volume
449
Issue
3
Start page
2794
Issn Identifier
0035-8711
Ads BibCode
2015MNRAS.449.2794D
Rights
open.access
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