{
  "bibcode": "2006GCN..5438....1G",
  "body": "M.R Goad, K.L. Page, O. Godet, P.T. O'Brien, S. Vaughan (U Leicester), and\nK. Hurley (Berkeley) report on behalf of the Swift-XRT team:\n\nAnalysis of 2.5 ks of Swift-XRT Photon Counting (PC) mode data taken\napproximately 31 hours after the latest outburst of SGR 1806-20 (Hurley et\nal., GCN Circ. 5419), reveals a dust-scattering halo, with a angular\nradius of approximately 120 arcseconds at this time.\n\nThe halo is also detected in 14 ks of XRT PC mode data taken 93.5 hours\nafter the burst. The measured size of the halo at this time, approximately\n212 arcseconds, is consistent with theoretical predictions for the\nexpansion of a dust scattering halo.\n\nThere are a number of well-known molecular clouds in the direction of SGR\n1806-20 (eg. Corbel et al. 1997, ApJ 478, 624), including the so-called\nexpanding arm located at a distance of approximately 3 kpc.\n\nUsing simple geometrical arguments (eg. Predehl et al. 2000, A&A, 357,\nL25), we place a firm upper limit on the distance to the dust scattering\nscreen of 6.6 kpc.  \n\nAdopting as a lower limit the estimated distance to SGR 1806-20 of 6 kpc\n(McClure-Griffiths and Gaensler 2005, ApJL 630, L161), then the dust\nscattering screen is located at approximately 3 kpc, consistent with an\norigin in the expanding Arm.  If, however, we adopt a distance of 15 kpc\nfor SGR 1806-20 (Corbel et al. 1997), then the dust scattering screen is\nlocated at ~4.5 kpc.\n\nAnother Soft Gamma-ray Repeater, SGR 1900+14, has previously shown a dust \nhalo (Kouveliotou et al. 2001, ApJ 558, L47).",
  "circularId": 5438,
  "createdOn": 1155313651000,
  "email": "kpa@star.le.ac.uk",
  "subject": "A dust-scattering halo detected around SGR 1806-20",
  "submitter": "Kim Page at U.of Leicester  <kpa@star.le.ac.uk>"
}