Constraints on the distance to SGR 1806-20 from HI absorption

Abstract

The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27 had a fluence more than 100 times higher than the only two other SGR flares ever recorded. Whereas the fluence is independent of distance, an estimate for the luminosity of the burst depends on the source's distance, which has previously been argued to be ~15 kpc. The burst produced a bright radio afterglow, against which Cameron et al. (2005) have measured an HI absorption spectrum. This has been used to propose a revised distance to SGR 1806-20 of between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum, and compare it both to HI emission data from the Southern Galactic Plane Survey and to archival 12-CO survey data. We confirm ~6 kpc, as a likely lower limit on the distance to SGR 1806-20, but argue that it is difficult to place an upper limit on the distance to SGR 1806-20 from the HI data currently available. The previous value of ~15 kpc thus remains the best estimate of the distance to the source.