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

TL;DR

This paper models the X-ray spectrum of GRO J1655-40 during its 2005 outburst, revealing element abundances and ionization conditions, and suggesting supernova enrichment in the system's evolution.

Contribution

It introduces synthetic spectral models for analyzing the X-ray spectrum, constraining element abundances, and understanding the warm absorber and outflow characteristics.

Findings

Lines are saturated or affected by scattering and partial coverage.

Ionization conditions are inconsistent with thermal wind driving.

Elemental abundances suggest enrichment by a core-collapse supernova.

Abstract

The spectrum from the black hole X-ray transient GRO J1655-40. obtained using the $Chandra$ High Energy Transmission Grating (HETG) 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 \cite{Mill06} and \cite{Mill08} 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 sub-solar values for elements lighter than calcium. Models show that this is consistent with enrichment by a core-collapse supernova. We discuss the implications these values for the evolutionary history of this system.