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 Kalpha 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 disk 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 disk. 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 disk radius is Rin = 17 +/- 5 Rg (where Rg is the Gravitational radius, GM/c^2), the emissivity dependence from the disk radius is -2.5 +/- 0.5, the inclination angle with respect to the line of sight is i = 43 +/- 5 degrees, and the outer radius of the emitting region in the disk is Rout > 200 Rg. We note that the accretion disk does not appear to be truncated at large radii, although the source is in a hard state at about 3 % 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 disk parameters found for the reflection component.

21 pages, 5 figures, 4 tables. To appear in Monthly Notices of the Royal Astronomical Society

References in corpus (10)

• Modelling the behaviour of accretion flows in X-ray binaries
• Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii
• Broad relativistic iron emission line observed in SAX J1808.4-3658
• XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658
• Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-ray Binary Serpens X-1
• A relativistically smeared spectrum in the neutron star X-ray Binary 4U 1705-44: Looking at the inner accretion disc with X-ray spectroscopy
• Disk reflection signatures in the spectrum of the bright Z-source GX 340+0
• BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state
• The first detection of Compton Reflection in the Low-Mass X-ray Binary 4U1705-44 with INTEGRAL and BeppoSAX
• Discovery of Red-Skewed K_alpha iron line in Cyg X-2 with Suzaku

Cited by in corpus (9)

• Accretion disks and coronae in the X-ray flashlight
• A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $\text{NuSTAR}$
• An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow
• Spectral and timing properties of IGR J00291+5934 during its 2015 outburst
• Disk reflection and a possible disk wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9-342058
• Study of the reflection spectrum of the LMXB 4U 1702-429
• NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus~X--2
• A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Ser~X-1
• Spectral analysis of the dipping LMXB system XB 1916-053