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 SGR 1806–20 magnetar .
From GNIRS 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-IR photometry from NIRI at Gemini North plus archival VLT/ISAAC datasets , we obtain a cluster distance modulus of 14.7 \pm 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 \pm 0.7 mag .
Together , our results favour a distance modulus of 14.7 \pm 0.4 mag ( 8.7 ^ { +1.8 } _ { -1.5 } 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 December 2004 giant flare is reduced by a factor of three to 7 \times 10 ^ { 46 } ergs ^ { -1 } , such that the contamination of BATSE short gamma ray bursts ( GRB ’ s ) from giant flares of extragalactic magnetars is reduced to a few percent .
We infer a magnetar progenitor mass of \sim 48 ^ { +20 } _ { -8 } M _ { \odot } , in close agreement with that obtained recently for the magnetar in Westerlund 1 .