Suzaku broad-band spectrum of 4U 1705-44: probing the reflection component in the hard state
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2015
- DOI:
- 10.1093/mnras/stv443
- arXiv:
- arXiv:1502.07669
- Bibcode:
- 2015MNRAS.449.2794D
- Keywords:
-
- line: formation;
- line: identification;
- stars: individual: 4U 1705-44;
- stars: neutron;
- X-rays: binaries;
- X-rays: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 21 pages, 5 figures, 4 tables. To appear in Monthly Notices of the Royal Astronomical Society