TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5438 SUBJECT: A dust-scattering halo detected around SGR 1806-20 DATE: 06/08/11 16:27:31 GMT FROM: Kim Page at U.of Leicester M.R Goad, K.L. Page, O. Godet, P.T. O'Brien, S. Vaughan (U Leicester), and K. Hurley (Berkeley) report on behalf of the Swift-XRT team: Analysis of 2.5 ks of Swift-XRT Photon Counting (PC) mode data taken approximately 31 hours after the latest outburst of SGR 1806-20 (Hurley et al., GCN Circ. 5419), reveals a dust-scattering halo, with a angular radius of approximately 120 arcseconds at this time. The halo is also detected in 14 ks of XRT PC mode data taken 93.5 hours after the burst. The measured size of the halo at this time, approximately 212 arcseconds, is consistent with theoretical predictions for the expansion of a dust scattering halo. There are a number of well-known molecular clouds in the direction of SGR 1806-20 (eg. Corbel et al. 1997, ApJ 478, 624), including the so-called expanding arm located at a distance of approximately 3 kpc. Using simple geometrical arguments (eg. Predehl et al. 2000, A&A, 357, L25), we place a firm upper limit on the distance to the dust scattering screen of 6.6 kpc. Adopting as a lower limit the estimated distance to SGR 1806-20 of 6 kpc (McClure-Griffiths and Gaensler 2005, ApJL 630, L161), then the dust scattering screen is located at approximately 3 kpc, consistent with an origin in the expanding Arm. If, however, we adopt a distance of 15 kpc for SGR 1806-20 (Corbel et al. 1997), then the dust scattering screen is located at ~4.5 kpc. Another Soft Gamma-ray Repeater, SGR 1900+14, has previously shown a dust halo (Kouveliotou et al. 2001, ApJ 558, L47).