* Astronomy

Title: Suzaku and BeppoSAX X-ray Spectra of the Persistently Accreting Neutron-Star Binary 4U 1705-44

Authors: Dacheng Lin (MIT, CESR), Ronald A. Remillard (MIT), Jeroen Homan (MIT)

We present an analysis of the broad-band spectra of 4U~1705--44 obtained with Suzaku in 2006--2008 and by BeppoSAX in 2000. The source exhibits two distinct states: the hard state shows emission from 1 to 150 keV, while the soft state is mostly confined to be <40 keV. We model soft-state continuum spectra with two thermal components, one of which is a multicolour accretion disk and the other is a single-temperature blackbody to describe the boundary layer, with additional weak Comptonisation represented by either a simple power law or the SIMPL model by Steiner et al. The hard-state continuum spectra are modelled by a single-temperature blackbody for the boundary layer plus strong Comptonisation, modelled by a cutoff power law. While we are unable to draw firm conclusions about the physical properties of the disk in the hard state, the accretion disk in the soft state appears to approximately follow L\propto T^{3.2}. The deviation from L\propto T^4, as expected from a constant inner disk radius, might be caused by a luminosity-dependent spectral hardening factor and/or real changes of the inner disk radius in some part of the soft state. The boundary layer apparent emission area is roughly constant from the hard to the soft states, with a value of about 1/11 of the neutron star surface. The magnetic field on the surface of the NS in 4U~1705--44 is estimated to be less than about 1.9 x 10^8 G, assuming that the disk is truncated by the ISCO or by the neutron star surface. Broad relativistic Fe lines are detected in most spectra and are modelled with the diskline model. The strength of the Fe lines is found to correlate well with the boundary layer emission in the soft state. In the hard state, the Fe lines are probably due to illumination of the accretion disk by the strong Comptonisation emission.

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