Chandra Observations of the Nuclear Star Cluster and Ultraluminous X-ray
  Sources in NGC 2139
  We report Chandra observations of the Scd galaxy NGC 2139, which is known to
host a recently formed (10^7.6 yrs) nuclear star cluster. The star cluster is
undetected in X-rays, with an upper bound on 0.5-7 keV luminosity of L_X < 7.1
x 10^37 erg/s. This bound implies a bolometric accretion luminosity <0.3
percent of the Eddington luminosity for a black hole with the mass
(approximately 3400 M_sun) expected from extrapolation of the M-sigma relation.
The lack of X-ray emission indicates that a black hole, if present, is not
undergoing significant accretion at the current time. While the central cluster
is undetected, the data reveal a substantial population of bright X-ray point
sources elsewhere in this galaxy, with eight qualifying as ultraluminous X-ray
sources with L_X > 10^39 erg/s. We use archival Hubble Space Telescope images
to identify candidate optical counterparts for seven Chandra sources, which in
most cases have optical luminosities and spatial profiles consistent with star
clusters. Compared with other galaxies, the number of luminous X-ray sources in
NGC 2139 is larger by a factor of 4 - 10 than expected based on its present
star formation rate and stellar mass. This finding can be understood if NGC
2139 has concluded a burst of star formation in the recent past, and suggests
that this galaxy could be important for testing the use of X-ray source
populations as a chronometer of star formation history.
