Phase Coherent Timing of RX J0806.3+1527 with ROSAT and Chandra

RX J0806.3+1527 is an ultracompact, double-degenerate binary with the shortest known orbital period (321.5 s). Hakala et al. have recently reported new optical measurements of the orbital frequency of the source indicating that the frequency has increased over the ≈9 yr since the earliest ROSAT observations. They find two candidate solutions for the long-term change in the frequency: ≈ 3 or 6 × 10^-16 Hz s^-1. Here we present the results of a phase coherent timing study of the archival ROSAT and Chandra data for RX J0806.3+1527 in the light of these new constraints. We find that the ROSAT-Chandra timing data are consistent with both of the solutions reported by Hakala et al. but that the higher = 6.1 × 10^-16 Hz s^-1 solution is favored at the ≈97% level. This large can be accommodated by an ≈1 M_☉ detached double-degenerate system powered in the X-ray by electrical energy (Wu et al.). With such a large , the system provides a unique opportunity to explore the interaction of gravitational radiation and electromagnetic torques on the evolution of an ultracompact binary.