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The Host Galaxy of GRB 031203: Implications of Its Low Metallicity, Low Redshift, and Starburst Nature

We present Keck/NIRSPEC near-IR images and Magellan/IMACS optical spectroscopy of the host galaxy of GRB 031203. The host is an actively star-forming galaxy at z = 0.1055 ± 0.0001. This is the lowest redshift GRB to date, aside from GRB 980425. From the hydrogen Balmer lines, we infer an extinction of AV = 3.62 ± 0.25 or a total reddening ET(B - V) = 1.17 ± 0.1 toward the sight line to the nebular regions. After correcting for reddening, we perform an emission-line analysis and derive an ISM temperature of T = 13400 ± 2000 K and electron density of ne = 300 cm-3. These imply a metallicity [O/H] = -0.72 ± 0.15 dex and a roughly solar abundance pattern for N, Ne, S, and Ar. Integrating Hα, we infer a dust-corrected star formation rate (SFR) of more than 11 M☉ yr-1. These observations have the following implications: (1) The galaxy has a low K'-band luminosity L ≈ L/5, typical of GRB host galaxies. (2) The low redshift indicates GRB 031203 had an isotropic-equivalent γ-ray energy release smaller than all previous confirmed GRB events. The burst discovery raises the likelihood of identifying many additional low-z, low-flux events with Swift. (3) The large SFR, low metallicity, and inferred hard radiation field are suggestive of massive star formation, supporting the collapsar model. (4) Several lines of evidence argue against the identification of GRB 031203 as an X-ray flash event.