# Suzaku and BeppoSAX X-ray Spectra of the Persistently Accreting Neutron-Star Binary 4U 1705-44 - Astrophysics > High Energy Astrophysical Phenomena

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Abstract: We present an analysis of the broad-band spectra of 4U~1705-44 obtained with{\it Suzaku} in 2006-2008 and by {\it BeppoSAX} in 2000. The source exhibitstwo distinct states: the hard state shows emission from 1 to 150 keV, while thesoft state is mostly confined to be $<40$ keV. We model soft-state continuumspectra with two thermal components, one of which is a multicolor accretiondisk and the other is a single-temperature blackbody to describe the boundarylayer, with additional weak Comptonization represented by either a simple powerlaw or the SIMPL model by Steiner et al. The hard-state continuum spectra aremodeled by a single-temperature blackbody for the boundary layer plus strongComptonization, modeled by a cutoff power law. While we are unable to draw firmconclusions about the physical properties of the disk in the hard state, theaccretion disk in the soft state appears to approximately follow $L\proptoT^{3.2}$. The deviation from $L\propto T^4$, as expected from a constant innerdisk radius, might be caused by a luminosity-dependent spectral hardeningfactor and-or real changes of the inner disk radius in some part of the softstate. The boundary layer apparent emission area is roughly constant from thehard to the soft states, with a value of about 1-11 of the neutron starsurface. The magnetic field on the surface of the NS in 4U~1705-44 isestimated to be less than about $1.9\times 10^8$ G, assuming that the disk istruncated by the ISCO or by the neutron star surface. Broad relativistic Felines are detected in most spectra and are modeled with the diskline model. Thestrength of the Fe lines is found to correlate well with the boundary layeremission in the soft state. In the hard state, the Fe lines are probably due toillumination of the accretion disk by the strong Comptonization emission.

Autor: Dacheng Lin MIT, CESR, Ronald A. Remillard MIT, Jeroen Homan MIT

Fuente: https://arxiv.org/