A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini Near-Infrared spectroscopy

We present H- and K- band spectroscopy of OB and Wolf -Rayet (WR) members of the Milky Way cluster 1806-20 (G10.0-0.3) to obtain a revised cluster distance, of relevance to the 2004 giant flare from the ( soft gamma repeater) SGR 1806-20 magnetar. From GNIRS ( Gemini Near-Infrared Spectrograph) spectroscopy obtained with Gemini South, four candidate OB stars are confirmed as late O/early B supergiants, while we support previous mid-WN and late WC classifications for two WR stars. Based upon an absolute K-s- band magnitude calibration for B supergiants and WR stars, and near- infrared ( IR) photometry from NIRI ( Near-Infrared Imager) at Gemini North plus archival VLT/ISAAC ( Very Large Telescope/ Infrared Spectrometer And Array Camera) data sets, we obtain a cluster distance modulus of 14.7 +/- 0.35 mag. The known stellar content of the 1806- 20 cluster suggests an age of 3 - 5 Myr, from which theoretical isochrone fits infer a distance modulus of 14.7 +/- 0.7 mag. Together, our results favour a distance modulus of 14.7 +/- 0.4 mag ( 8.7(-1.5)(+ 1.8) kpc) to the 1806- 20 cluster, which is significantly lower than the nominal 15 kpc distance to the magnetar. For our preferred distance, the peak luminosity of the 2004 December giant flare is reduced by a factor of 3 to 7 x 1046 erg s(-1), such that the contamination of BATSE ( Burst And Transient Source Experiment) short gamma- ray bursts ( GRBs) from giant flares of extragalactic magnetars is reduced to a few per cent. We infer a magnetar progenitor mass of similar to 48(-8)(+20) M-circle dot, in close agreement with that obtained recently for the magnetar in Westerlund 1.