Transient Low-Mass X-Ray Binary Populations in Elliptical Galaxies NGC
  3379 and NGC 4278
  We propose a physically motivated and self-consistent prescription for the
modeling of transient neutron star (NS) low-mass X-ray binary (LMXB)
properties, such as duty cycle (DC), outburst duration and recurrence time. We
apply this prescription to the population synthesis (PS) models of field LMXBs
presented by Fragos et al. (2008), and compare the transient LMXB population to
the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278,
which revealed several transient sources (Brassington et al., 2008, 2009). We
are able to exclude models with a constant DC for all transient systems, while
models with a variable DC based on the properties of each system are consistent
with the observed transient populations. We predict that the majority of the
observed transient sources in these two galaxies are LMXBs with red giant
donors. Our comparison suggests that LMXBs formed through evolution of
primordial field binaries are dominant in globular cluster (GC) poor elliptical
galaxies, while they still have a significant contribution in GC rich ones.
