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J. A. Irwin, J. N. Bregman (University of Michigan)
We present joint archival ROSAT PSPC + ASCA spectra of the X-ray faint early-type galaxy NGC4697. Unlike massive, X-ray bright early-type galaxies whose X-ray emission is primarily from hot gas with a temperature of 0.5-1.5 keV, X-ray faint early-type galaxies have X-ray spectra characterized by a hard (5--10 keV) plus a very soft (0.2-0.4 keV) thermal component. The hard component is generally attributed to the integrated emission from low-mass X-ray binaries (LMXBs), while the soft component is attributable to either a warm interstellar medium or the same collection of LMXBs responsible for the hard emission. Our spectral study of NGC4697 reveals that a hard + soft model describes the spectra best, consistent with other X-ray faint early-type galaxies. However, the soft component in this galaxy is stronger relative to the hard component than is expected. It is possible that the soft component in this galaxy emanates from both a warm ISM and LMXBs, leading to a stronger soft component than in galaxies with simply emission from LMXBs. A long archival ROSAT HRI image of the galaxy seems to support this claim. We calculate the relative contributions from point sources and diffuse emission in the HRI image, and discuss the implications of the importance of each component to the X-ray emission from early-type galaxies in general.
This work is funded through a grant provided as part of the Chandra Fellowship Program.