ar X iv : a st ro - p h / 01 09 36 1 v 2 1 M ay 2 00 2 Large Torque Variations in Two Soft Gamma Repeaters

We have monitored the pulse frequencies of the two soft gamma repeaters SGR 1806−20 and SGR 1900+14 through the beginning of year 2001 using primarily Rossi X-ray Timing Explorer Proportional Counter Array observations. In both sources, we observe large changes in the spin-down torque up to a factor of ∼4, which persist for several months. Using long baseline phase-connected timing solutions as well as the overall frequency histories, we construct torque noise power spectra for each SGR. The power spectrum of each source is very red (power-law slope ∼ −3.5). The torque noise power levels are consistent with some accreting systems on time scales of ∼1 year, yet the full power spectrum is much steeper in frequency than any known accreting source. To the best of our knowledge, torque noise power spectra with a comparably steep frequency dependence have only been seen in young, glitching radio pulsars (e.g. Vela). The observed changes in spin-down rate do not correlate with burst activity, therefore, the physical mechanisms behind each phenomenon are also likely unrelated. Within the context of the magnetar model, seismic activity cannot account for both the bursts and the long-term torque changes unless the seismically active regions are decoupled from one another. Subject headings: stars: individual (SGR 1900+14) — stars: individual (SGR 1806-20) — stars: pulsars — X-rays: bursts Universities Space Research Association; [email protected] National Space Science and Technology Center, 320 Sparkman Dr. Huntsville, AL 35805 NASA Marshall Space Flight Center Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899 NASA Goddard Space Flight Center, Greenbelt, MD 20771 University of California at Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450 Astronomical Institute “Anton Pannekoek” and CHEAF, University of Amsterdam, 403 Kruislaan, 1098 SJ Amsterdam, NL