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

We present Hand 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 Ks-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±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+1.8 −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 × 1046 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 ∼ 48+20 −8 M⊙, in close agreement with that obtained recently for the magnetar in Westerlund 1.