The Host Galaxy of GRB 980703 at Radio Wavelengths—a Nuclear Starburst in an Ultraluminous Infrared Galaxy

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Abstract
We present radio observations of GRB 980703 at 1.43, 4.86, and 8.46 GHz for the period of 350-1000 days after the burst. These radio data clearly indicate that there is a persistent source at the position of GRB 980703 with a flux density of approximately 70 μJy at 1.43 GHz and a spectral index β ≈ 0.32, where Fν ∝ ν-β. We show that emission from the afterglow of GRB 980703 is expected to be 1-2 orders of magnitude fainter and therefore cannot account for these observations. We interpret this persistent emission as coming from the host galaxy—the first example of a γ-ray burst (GRB) host detection at radio wavelengths. We find that it can be explained as a result of a star formation rate (SFR) of massive stars (M > 5 M☉) of ≈140 M☉ yr-1, which gives a total SFR (0.1 M☉ < M < 100 M☉) of ≈750 M☉ yr-1. On the basis of these data alone we cannot rule out that some fraction of the radio emission originates from an obscured active galactic nucleus. Using the correlation between the radio and far-IR (FIR) luminosities of star-forming galaxies, we find that the host of GRB 980703 is at the faint end of the class of ultraluminous infrared galaxies, with LFIR ~ 1012 L☉. From the radio measurements of the offset between the burst and the host and of the size of the host, we conclude that GRB 980703 occurred near the center of the galaxy in a region of star formation. A comparison of the properties of this galaxy with radio and optical surveys at a similar redshift (z ≈ 1) reveals that the host of GRB 980703 is an average radio-selected star-forming galaxy. This result has significant implications for the potential use of a GRB-selected galaxy sample for the study of galaxies and the intergalactic medium at high redshifts, especially using radio observations, which are insensitive to extinction by dust and provide an unbiased estimate of the SFR through the well-known radio-FIR correlation.
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