  47 Tuc X9 is a low mass X-ray binary (LMXB) in the globular cluster 47
Tucanae, and was previously thought to be a cataclysmic variable. However,
Miller-Jones et al. (2015) recently identified a radio counterpart to X9
(inferring a radio/X-ray luminosity ratio consistent with black hole LMXBs),
and suggested that the donor star might be a white dwarf. We report
simultaneous observations of X9 performed by Chandra, Nustar and Australia
Telescope Compact Array. We find a clear 28.18$\pm$0.02 min periodic modulation
in the Chandra data, which we identify as the orbital period, confirming this
system as an ultracompact X-ray binary. Our X-ray spectral fitting provides
evidence for photoionized gas having a high oxygen abundance in this system,
which indicates a C/O white dwarf donor. We also identify reflection features
in the hard X-ray spectrum, making X9 the faintest LMXB to show X-ray
reflection. We detect a $\sim$ 6.8 day modulation in the X-ray brightness by a
factor of 10, in archival Chandra, Swift, and Rosat data. The simultaneous
radio/X-ray flux ratio is consistent with either a black hole primary or a
neutron star primary, if the neutron star is a transitional millisecond pulsar.
Considering the measured orbital period (with other evidence of a white dwarf
donor), and the lack of transitional millisecond pulsar features in the X-ray
light curve, we suggest that this could be the first ultracompact black hole
X-ray binary identified in our Galaxy.
