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RX J0806.3+1527 is a candidate double degenerate binary with possibly theshortest known orbital period. The source shows an 100% X-ray intensitymodulation at the putative orbital frequency of 3.11 mHz (321.5 s). If thesystem is a detached, ultracompact binary gravitational radiation should drivespin-up with a magnitude of ~10-16 Hz/s. Here we describe the results of thefirst 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 spacedover 320 days. These data conclusively show that the X-ray frequency isincreasing at a rate of 3.77 +- 0.8 x 10-16 Hz/s. Using our new ephemeris weare able to phase up all the earlier Chandra and ROSAT data and show they areconsistent with a rate of 3.63 +- 0.06 x 10-16 Hz/s over the past decade. Thisvalue appears consistent with that recently derived by Israel et al. largelyfrom monitoring of the optical modulation, and is in rough agreement with thesolutions reported initially by Hakala et al., based on ground-based opticalobservations. The large spin-up is consistent with gravitational radiationlosses driving the evolution. An intermediate polar (IP) scenario where theobserved X-ray period is the spin period of an accreting white dwarf appearsless tenable. If the ultracompact scenario is correct, then the X-ray fluxcannot be powered by stable accretion which would drive the components apart,suggesting a new type of energy source (perhaps electromagnetic) may power theX-ray flux.Comment: 23 pages, 9 figures, AASTeX, accepted for publication in the  Astrophysical Journa

Precision X‐Ray Timing of RX J0806.3+1527 withChandra: Evidence for Gravitational Radiation from an Ultracompact Binary