The Spectral and Temporal Properties of Transient Sources in Early-Type
  Galaxies
  We report the spectral and temporal variability properties of 18 candidate
transient and potential transient (TC and PTC) sources detected in deep
multi-epoch Chandra observation of the nearby elliptical galaxies, NGC 3379,
NGC 4278 and NGC 4697. Only one source can be identified with a background
counterpart, leaving 17 TCs + PTCs in the galaxies. Of these, 14 are in the
galaxy field, supporting the theoretical picture that the majority of field
X-ray binaries (XRBs) will exhibit transient accretion for >75% of their
lifetime. Three sources are coincident with globular clusters (GCs), including
two high-luminosity candidate black hole (BH) XRBs, with Lx=5.4E38 erg/s, and
Lx=2.8E39 erg/s, respectively. The spectra, luminosities and temporal behavior
of these 17 sources suggest that the transient population is heterogeneous,
including neutron star (NS) and BH XRBs in both normal and high-rate accretion
modes, and super soft sources containing white dwarf binaries. Our TC and PTC
detections are noticeably fewer that the number expected from the populations
synthesis (PS) models of Fragos et al. (2009), tailored to our new Chandra
pointings of NGC 4278. We attribute this discrepancy to the PS assumption that
the transient population is composed of NS XRBs, as well as differences between
the statistical analysis and error estimates used in the model and our
observations.
