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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) membersof the Milky Way cluster 1806-20 (G10.0-0.3) to obtain a revised clusterdistance, of relevance to the 2004 giant flare from the (soft gammarepeater) SGR1806-20 magnetar. From GNIRS (Gemini Near-InfraredSpectrograph) spectroscopy obtained with Gemini South, four candidate OBstars are confirmed as late O/early B supergiants, while we supportprevious mid-WN and late WC classifications for two WR stars. Based uponan absolute Ks-band magnitude calibration for B supergiantsand WR stars, and near-infrared (IR) photometry from NIRI (Near-InfraredImager) at Gemini North plus archival VLT/ISAAC (Very LargeTelescope/Infrared Spectrometer And Array Camera) data sets, we obtain acluster distance modulus of 14.7 +/- 0.35mag. The known stellar contentof the 1806-20 cluster suggests an age of 3-5Myr, from which theoreticalisochrone fits infer a distance modulus of 14.7 +/- 0.7mag. Together,our results favour a distance modulus of 14.7 +/- 0.4mag(8.7+1.8-1.5kpc) to the 1806-20 cluster, which issignificantly lower than the nominal 15kpc distance to the magnetar. Forour preferred distance, the peak luminosity of the 2004 December giantflare is reduced by a factor of 3 to 7 ×1046ergs-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 percent. We infer a magnetar progenitor mass of~48+20-8Msolar, in close agreement withthat obtained recently for the magnetar in Westerlund 1.
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