We have phase connected a sequence of Rossi X-ray Timing Explorer Proportional Counter Array observations of SGR 1806 - 20 covering 178 days .
We find a simple secular spin-down model does not adequately fit the data .
The period derivative varies gradually during the observations between 8.1 and 11.7 \times 10 ^ { -11 } s s ^ { -1 } ( at its highest , \sim 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 phase 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 radio pulsars ( \Delta _ { 8 } = 4.8 ; frequency derivative average power \approx 7 \times 10 ^ { -20 } cyc ^ { 2 } s ^ { -3 } ) .
Combining these results with the noise levels measured for some AXPs , we find all magnetar candidates have \Delta _ { 8 } values larger than those expected from a simple extrapolation of the correlation found in radio pulsars .
We find that the timing noise in SGR 1806 - 20 is greater than or equal to the levels found in some accreting systems ( e.g. , Vela X - 1 , 4U 1538 - 52 and 4U 1626 - 67 ) , but the spin-down of SGR 1806 - 20 has thus far maintained coherence over 6 years .
Alternatively , an orbital model with a period P _ { orb } = 733 days provides a statistically acceptable fit to the data .
If the phase residuals are created by Doppler shifts from a gravitationally bound companion , then the allowed parameter space for the mass function ( small ) and orbital separation ( large ) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR .