Mon. Not. R. Astron. Soc., 386, L23-L27 (2008/May-1)
A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini Near-Infrared Spectroscopy.
BIBBY J.L., CROWTHER P.A., FURNESS J.P. and CLARK J.S.
Abstract (from CDS):
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) SGR1806-20 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 Ks-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±0.35mag. The known stellar content of the 1806-20 cluster suggests an age of 3-5Myr, from which theoretical isochrone 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 is significantly lower than the nominal 15kpc 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 7x1046erg/s, 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 ∼48+20–8Msolar, in close agreement with that obtained recently for the magnetar in Westerlund 1.
©2008 The Authors. Journal compilation © 2008 RAS
stars: early-type - pulsars: individual: SGR1806-20 - stars: Wolf-Rayet - Galaxy: kinematics and dynamics - open clusters and associations: individual: 1806-20
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