The Ages of High Mass X-ray Binaries in NGC 2403 and NGC 300
  We have examined resolved stellar photometry from HST imaging surrounding 18
high-mass X-ray binary (HMXB) candidates in NGC 300 and NGC 2403 as determined
from combined Chandra/HST analysis. We have fit the color-magnitude
distribution of the surrounding stars with stellar evolution models. All but
one region in NGC 300 and two in NGC 2403 contain a population with an age
between 20 and 70 Myr. One of the candidates is the ultraluminous X-ray source
(ULX) in NGC 2403, which we associate with a 60 Myr old population. These age
distributions provide additional evidence that 16 of these 18 candidates are
HMXBs. Furthermore, our results suggest that the most common HMXB age in these
galaxies is 40-55 Myr. This preferred age is similar to observations of HMXBs
in the Small Magellanic Cloud, providing new evidence of this formation
timescale, but in higher metallicity populations. We suggest that this
preferred HMXB age is the result of the fortuitous combination of two physical
effects. First, this is the age of a population when the greatest rate of
core-collapse events should be occurring, maximizing neutron star production.
Second, this is the age when B stars are most likely to be actively losing
mass. We also discuss our results in the context of HMXB feedback in galaxies,
confirming HMXBs as a potentially important source of energy for the
interstellar medium in low-mass galaxies.
