Spectrum Synthesis Modeling of the X-Ray Spectrum of GRO J1655-40 Taken During the 2005 Outburst

Abstract

The spectrum from the black hole X-ray transient GRO J1655-40 obtained using the Chandra High Energy Transmission Grating in 2005 is notable as a laboratory for the study of warm absorbers, and for the presence of many lines from odd-Z elements between Na and Co ( and Ti and Cr) not previously observed in X-rays. We present synthetic spectral models which can be used to constrain these element abundances and other parameters describing the outflow from the warm absorber in this object. We present results of fitting to the spectrum using various tools and techniques, including automated line fitting, phenomenological models, and photoionization modeling. We show that the behavior of the curves of growth of lines from H-like and Li-like ions indicate that the lines are either saturated or affected by filling-in from scattered or a partially covered continuum source. We confirm the conclusion of previous work by Miller et al., which shows that the ionization conditions are not consistent with wind driving due to thermal expansion. The spectrum provides the opportunity to measure abundances for several elements not typically observable in the X-ray band. These show a pattern of enhancement for iron peak elements, and solar or subsolar values for elements lighter than calcium. Models show that this is consistent with enrichment by a core-collapse supernova. We discuss the implications of these values for the evolutionary history of this system.