0 Timing Noise in SGR 1806 - 20

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 × 10−11 s s−1 (at its highest, ∼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 (∆8 = 4.8; frequency derivative average power ≈ 7 × 10−20 cyc2 s−3). Combining these results with the noise levels measured for some AXPs, we find all magnetar candidates have ∆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 Porb = 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. Subject headings: stars: individual (SGR 1806-20) — stars: pulsars — X-rays: bursts Universities Space Research Association; [email protected] NASA Marshall Space Flight Center, SD50, Huntsville, AL 35812 Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899 Department of Physics and Astronomy, University of North Carolina, Philips Hall, Chapel Hill, NC 27599-3255 Department of Astronomy, University of Texas, RLM 15.308, Austin, TX 78712-1083 University of California at Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450 NASA Goddard Space Flight Center, Greenbelt, MD 20771 Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara-shi, Kanagawa 229, Japan