2006ApJ...650..879F

We report recent Chandra observations of the ULX in the elliptical galaxy NGC 3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ∼5 years ago, we measure a flux modulation with a period of ∼12.6 hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC 3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient; however, historical X-ray sampling suggests that the current ``on'' phase has lasted ∼10 yr. We discuss our results in terms of ADC and wind-feedback models. If the flux modulation is orbital, we can constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4 M_☉and 12.5-17 hr, respectively. The duration of the mass transfer phase so far is probably ∼1 Gyr, and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.