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R. Kilgard, A. Prestwich, A. Zezas, P. Kaaret, M. Raley, M. Krauss, J. McDowell (SAO)
Ultra Luminous X-ray Sources are extragalactic X-ray sources where the X-ray luminosity is above that of the Eddington luminosity for a solar mass compact object, but less than that expected from an AGN. The luminosity of the most spectacular example of an ULX in M82 is close to 1041ergs s-1. These objects have been interpreted as intermediate mass black holes (i.e. ~1000M\odot) and highly luminous beamed microquasars. The latter explanation requires that ULX's are short-lived, and be found preferentially in starburst galaxies. We report on a ``minisurvey" of 3 starburst galaxies (the Antennae, M82, NGC 253) and 3 non-starburst spiral galaxies (NGC 3184, 1291 and 2681). The goal of this work is to determine whether there is a statistical excess of ULX's in starburst galaxies compared to normal spirals. We show that the Antennae has clear evidence for an excess of high luminosity objects compared to the spirals. In contrast, the luminosity function for M82 is only slightly skewed towards higher luminosities and NGC 253 shows no evidence for an excess compared to the spirals. We discuss whether these differences relate to the evolutionary state of the star formation, or are a ``variability conspiracy".