2004ApJ...610..416S

V407 Vul (RX J1914.4+2456) is a candidate double-degenerate binary with a putative 1.756 mHz (9.5 minute) orbital frequency. In a previous timing study using archival ROSAT and ASCA data, we reported evidence for an increase of this frequency at a rate consistent with expectations for gravitational radiation from a detached ultracompact binary system. Here we report the results of new Chandra timing observations, which confirm the previous indications of spin-up of the X-ray frequency and provide much tighter constraints on the frequency derivative, ν{dot}. We obtained with Chandra a total of 90 ks of exposure in two epochs separated in time by 11.5 months. The total time span of the archival ROSAT, ASCA, and new Chandra data is now ~10.5 yr. This more than doubles the interval spanned by the ROSAT and ASCA data alone, providing much greater sensitivity to a frequency derivative. With the addition of the Chandra data, an increasing frequency is evident, and the mean ν{dot} is (7.0±0.8)x10^–18 Hz/s. Although a long-term spin-up trend is confirmed, there is excess variance in the phase timing residuals, perhaps indicating shorter timescale torque fluctuations or phase instability associated with the source of the X-ray flux. Power spectral searches for periods longward of the 9.5 minute period do not find any significant modulations; however, the sensitivity of searches in this frequency range are somewhat compromised by the dithering of the Chandra attitude. The observed spin-up is of a magnitude consistent with that expected from gravitational radiation decay; however, the factor of ~3 variations in flux combined with the timing noise could conceivably result from accretion-induced spin-up of a white dwarf. Continued monitoring to explore correlations of torque with X-ray flux could provide a further test of this hypothesis.