  RX J0806.3+1527 is a candidate double degenerate binary with possibly the
shortest known orbital period. The source shows an 100% X-ray intensity
modulation at the putative orbital frequency of 3.11 mHz (321.5 s). If the
system is a detached, ultracompact binary gravitational radiation should drive
spin-up with a magnitude of ~10-16 Hz/s. Here we describe the results of the
first phase coherent X-ray monitoring campaign on RX J0806.3+1527 with Chandra.
We obtained a total of 70 ksec of exposure in 6 epochs logarithmically spaced
over 320 days. These data conclusively show that the X-ray frequency is
increasing at a rate of 3.77 +- 0.8 x 10-16 Hz/s. Using our new ephemeris we
are able to phase up all the earlier Chandra and ROSAT data and show they are
consistent with a rate of 3.63 +- 0.06 x 10-16 Hz/s over the past decade. This
value appears consistent with that recently derived by Israel et al. largely
from monitoring of the optical modulation, and is in rough agreement with the
solutions reported initially by Hakala et al., based on ground-based optical
observations. The large spin-up is consistent with gravitational radiation
losses driving the evolution. An intermediate polar (IP) scenario where the
observed X-ray period is the spin period of an accreting white dwarf appears
less tenable. If the ultracompact scenario is correct, then the X-ray flux
cannot be powered by stable accretion which would drive the components apart,
suggesting a new type of energy source (perhaps electromagnetic) may power the
X-ray flux.
