[vsnet-grb-info 3604] A dust-scattering halo detected around SGR 1806-20

[GCN Circulars]

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  <kpa at star.le.ac.uk>

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).