A Variable Black Hole X-Ray Source in a NGC 1399 Globular Cluster
  We have discovered an accreting black hole (BH) in a spectroscopically
confirmed globular cluster (GC) in NGC 1399 through monitoring of its X-ray
activity. The source, with a peak luminosity of L_x=2x10^39 ergs/s, reveals an
order of magnitude change in the count rate within ~10 ks in a Chandra
observation. The BH resides in a metal-rich [Fe/H]~0.2 globular cluster. After
RZ2109 in NGC 4472 this is only the second black-hole X-ray source in a GC
confirmed via rapid X-ray variability. Unlike RZ2109, the X-ray spectrum of
this BH source did not change during the period of rapid variability. In
addition to the short-term variability the source also exhibits long-term
variability. After being bright for at least a decade since 1993 within a span
of 2 years it became progressively fainter, and eventually undetectable, or
marginally detectable, in deep Chandra and XMM observations. The source also
became harder as it faded. The characteristics of the long term variability in
itself provide sufficient evidence to identify the source as a BH. The long
term decline in the luminosity of this object was likely not recognized in
previous studies because the rapid variability within the bright epoch
suppressed the average luminosity in that integration. The hardening of the
spectrum accompanying the fading would also make this black hole source
indistinguishable from an accreting neutron star in some epochs. Therefore some
low mass X-ray binaries identified as NS accretors in snapshot studies of
nearby galaxies may also be BHs. Thus the discovery of the second confirmed BH
in an extragalactic GC through rapid variability at the very least suggests
that accreting BHs in GCs are not exceedingly rare occurences.
