Fading of the X-ray flux from the black hole in the NGC 4472 globular
  cluster RZ 2109
  We present the results of new X-ray observations of XMMU 122939.7+075333, the
black hole (BH) in the globular cluster RZ 2109 in the Virgo Cluster galaxy NGC
4472. A combination of non-detections and marginal detections in several recent
Swift and Chandra observations show that the source has varied by at least a
factor of 20 in the past 6 years, and that the variations seem not just to be
"flickering." This variation could be explained with changes in the absorption
column intrinsic to the source no larger than those which were previously seen
near the peak of the 1989 outburst of the Galactic BH X-ray binary V404 Cyg.
The large amplitude variations are also a natural expectation from a
hierarchical triple system with Kozai cycles -- the mechanism recently proposed
to produce BH-white dwarf (WD) binaries in globular clusters. On the other
hand, variation by such a large factor on timescales of years, rather than
centuries, is very difficult to reconcile with the scenario in which the X-ray
emission from XMMU 122939.7+075333 is due to fallback of material from a
tidally destroyed or detonated WD.
