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

TL;DR

This study refines the distance to the 1806-20 star cluster using near-infrared spectroscopy, impacting the estimated luminosity of its magnetar flare and progenitor mass, with implications for understanding extragalactic magnetar flares.

Contribution

It provides a revised, more accurate distance measurement to the 1806-20 cluster using spectroscopy and photometry, challenging previous estimates and refining magnetar-related parameters.

Findings

Revised cluster distance of 8.7 kpc, lower than previous 15 kpc estimate.

Reduced peak luminosity of the 2004 giant flare to 7 x 10^46 erg/s.

Magnetar progenitor mass estimated at ~48 solar masses.

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 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 X 10^46 erg/s, 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 Msun, in close agreement with that obtained recently for the magnetar in Westerlund 1.