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A.H. Prestwich, R. Kilgard, M.I. Krauss, P. Kaaret, A. Zezas, J.C McDowell, F. Primini (Harvard-Smithsonian CfA), M. Ward (U of Leicester, UK), G. Trinchieri (Osservatorio Astronomico di Brera)
Nearby spiral galaxies contain a multitude of X-ray sources beautifully resolved by Chandra. In disk galaxies the X-ray sources follow the spiral arms, however in bulges they tend to cluster around a central source. By analogy with X-ray sources in the Milky Way and Local Group galaxies, we would expect the bulge sources to be low mass X-ray binaries and the sources in disk galaxies to be high mass X-ray binaries and possibly supernova remnants. This is consistent with what is seen, but is yet to be confirmed by Chandra.
We examine the X-ray colors of sources in a sample of nearby spiral galaxies observed with Chandra. We look for differences in the X-ray colors of bulge and disk sources which may be indicative of different classes of binaries. Preliminary results show that the X-ray colors of many sources (in both bulges and disks) are consistent with a power law spectrum with absorption. Several sources have very hard X-ray colors and may be background sources, or possibly black hole candidates in a low, hard, state. We also find a population of soft sources in disk galaxies. These soft sources may be harder to detect in spiral bulges were there is more diffuse emission, or they may be genuinely under-abundant in bulges compared to disks. The implications of these results will be discussed.
Support for this program comes from the Chandra X-ray Center under NASA Contract NAS8-39073 and GO1-2092A.