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 \pm 5 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 .