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We present 23 candidate X-ray binaries with luminosities down to 1.8x10^36erg/s, in the nearby starburst galaxy NGC 4449, from observations totaling 105ksec taken with the ACIS-S instrument on the Chandra Space Telescope. Wedetermine count rates, luminosities, and colors for each source, and performspectral fits for sources with sufficient counts. We also compile a new catalogof 129 compact star clusters in NGC 4449 from high resolution, multi-bandoptical images taken with the Hubble Space Telescope, doubling the number ofclusters known in this galaxy. The UBVI,Ha luminosities of each cluster arecompared with predictions from stellar evolution models to estimate their agesand masses. We find strong evidence for a population of very young massive,black-hole binaries, which comprise nearly 50% of the detected X-ray binariesin NGC 4449. Approximately a third of these remain within their parent starclusters, which formed t < 6-8 Myr ago, while others have likely been ejectedfrom their parent clusters. We also find evidence for a population of somewhatolder X-ray binaries, including both supergiant and Be-binaries, which appearto be associated with somewhat older t ~ 100-400 Myr star clusters, and oneX-ray binary in an ancient (t ~ 10 Gyr) globular cluster. Our results suggestthat detailed information on star clusters can significantly improveconstraints on X-ray binary populations in star-forming galaxies.

The Connection Between X-ray Binaries and Star Clusters in NGC 4449