Elemental Abundances of Nearby Galaxies through High Signal-to-Noise Ratio XMM-Newton Observations of Ultraluminous X-Ray Sources

Abstract

In this paper we examined XMM-Newton EPIC spectra of 14 ultraluminous X-ray sources (ULXs) in addition to the XMM-Newton RGS spectra of two sources (Holmberg II X-1 and Holmberg IX X-1). We determined oxygen and iron abundances of the host galaxy's ISM using K-shell (O) and L-shell (Fe) X-ray photoionization edges toward these ULXs. We found that the oxygen abundances closely matched recent solar abundances for all of our sources, implying that ULXs live in similar local environments despite the wide range of galaxy host properties. Also, we compare the X-ray hydrogen column densities (n_H) for eight ULXs with column densities obtained from radio H I observations. The X-ray model n_H values are in good agreement with the H I n_H values, implying that the hydrogen absorption toward the ULXs is not local to the source (with the exception of the source M81 XMM1). In order to obtain the column density and abundance values, we fitted the X-ray spectra of the ULXs with a combined power law and one of several accretion disk models. We tested the abundances obtained from the XSPEC models bbody, diskbb, grad, and diskpn along with a power law, finding that the abundances were independent of the thermal model used. We comment on the physical implications of these different model fits. We also note that very deep observations allow a breaking of the degeneracy noted by Stobbart and coworkers favoring a high-mass solution for the absorbed grad+power-law model.