Precise Chandra localization of the soft gamma-ray repeater SGR 1806-20

We present observations of the soft gamma-ray repeater SGR 1806-20 taken with the Chandra X-Ray Observatory. We identify the X-ray counterpart of SGR 1806[20 based on detection of 7.5 s pulsations. Using three unrelated X-ray sources (and USNO stars) as position references, we are able to determine that the SGR is at alpha(2000) = 18(h)08(m)39(8).32 and delta(2000) = -20degrees24'39."5 with rms uncertainty of 0."3 in each coordinate. We find that SGR 1806-20 is located within the 1 sigma error region determined by Interplanetary Network data and is 14" +/- 0."5 distant from the nonthermal core of SNR G10.0-0.3, excluding SGR 1806-20 as the origin of the core. We see evidence of a significant deviation of the spin-down of SGR 1806-20 from its long-term trend, demonstrating erratic spin-down behavior in this source similar to that seen in other SGRs. Finally, we show that there is a broad X-ray halo surrounding SGR 1806-20 out to radii similar to1' due to scattering in the interstellar medium.