A Broad Iron Line in the Chandra High Energy Transmission Grating Spectrum of 4U 1705-44

Abstract

We present the results of a Chandra 30 ks observation of the low-mass X-ray binary and atoll source 4U 170544. Here we concentrate on the study of discrete features in the energy spectrum at energies below ∼3 keV, as well as on the iron Ka line, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV lines are probably identified with Lya emission from Mg xii and S xvi, respectively. The identification of the feature at ∼2.0 keV is uncertain because of the presence of an instrumental feature at the same energy. The iron Ka line at ∼6.5 keV is found to be intrinsically broad (keV); its width can be explained FWHM ∼ 1.2 by reflection from a cold accretion disk extending down to ∼15 km from the neutron star center or by Compton broadening in the external parts of a hot (∼2 keV) Comptonizing corona. We finally report here precise X-ray coordinates of the source.