A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini Near-Infrared Spectroscopy
Abstract
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 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 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2008
- DOI:
- 10.1111/j.1745-3933.2008.00453.x
- arXiv:
- arXiv:0802.0815
- Bibcode:
- 2008MNRAS.386L..23B
- Keywords:
-
- stars: early-type;
- pulsars: individual: SGR1806-20;
- stars: Wolf-Rayet;
- Galaxy: kinematics and dynamics;
- open clusters and associations: individual: 1806-20;
- Astrophysics
- E-Print:
- 6 pages, 4 figures, accepted for MNRAS Letters