Timing noise in SGR 1806-20

We have phase-connected a sequence of Rossi X-Ray Timing Explorer Proportio nal Counter Array observations of SGR 1806-20 covering 178 days. We find th at a simple secular spin-down model does not adequately fit the data. The p eriod derivative varies gradually during the observations between 8.1 x 10( -11) and 11.7 x 10(-11) s s(-1) (at its highest, similar to 40% larger than the long-term trend), while the average burst rate as seen with the Burst and Transient Source Experiment drops throughout the time interval. The pha se residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in ra dio pulsars (Delta(8) = 4.8; frequency derivative average power approximate to 7 x 10(-20) cycles(2) s(-3)). Combining these results with the noise le vels measured for some anomalous X-ray pulsars, we find that all magnetar c andidates have Delta(8) values larger than those expected from a simple ext rapolation of the correlation found in radio pulsars. We find that the timi ng noise in SGR 1806-20 is greater than or equal to the levels found in som e accreting systems (e.g,, Vela X-l, 4U 1538-52, and 4U 1626-67), but the s pin-down of SGR 1806-20 has thus far maintained coherence over 6 yr. Altern atively, an orbital model with a period P-orb = 733 days provides a statist ically acceptable fit to the data. If the phase residuals are created by Do ppler shifts from a gravitationally bound companion, then the allowed param eter space for the mass function (small) and orbital separation (large) rul e out the possibility of accretion from the companion sufficient to power t he persistent emission from the SGR.