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Ultracompact X-ray binaries (UCXBs) are thought to have relatively simplebinary evolution post-contact, leading to clear predictions of their luminosityfunction. We test these predictions by studying the long-term behavior of knownUCXBs in our Galaxy, principally using data from the MAXI All-Sky Survey andthe Galactic Bulge Scans with RXTE's PCA instrument. Strong luminosityvariations are common (and well-documented) among persistent UCXBs, whichrequires an explanation other than the disk instability mechanism. We measurethe luminosity function of known UCXBs in the Milky Way, which extends to lowerluminosities than some proposed theoretical luminosity functions of UCXBs. Thedifference between field and globular cluster X-ray luminosity functions inother galaxies cannot be explained by an increased fraction of UCXBs inglobular clusters. Instead, our measured luminosity function suggests thatUCXBs only make up a small fraction of the X-ray binaries above a few times10^36 ergs/s in both old field populations and globular clusters.

Galactic Ultracompact X-ray Binaries: Empirical Luminosities