The time-variable ultraluminous X-ray sources of the Antennae

We report the first results of the Chandra temporal monitoring of the ultraluminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039). Observations at four different epochs, covering timescales of 2 yr to 2 months, show variability in seven out of nine ULXs, confirming that they are likely to be accreting compact X-ray binaries. The seven variable ULXs exhibit a variety of temporal and spectral behaviors: one has harder X-ray colors with decreasing luminosity, similar to the black hole binary Cyg X-1, but four other ULXs show the opposite behavior. We suggest that the latter may be black hole binaries accreting at very high rates.