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Author:
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Levesque, Emily M.; Bloom, Joshua S.; Butler, Nathaniel R.; Perley, Daniel; Cenko, S. Bradley; Prochaska, Xavier; Kewley, Lisa; Bunker, Andrew; Chen, Hsiao-Wen; Chornock, Ryan; Filippenko, Alexei V.; Glazebrook, Karl; López, Sebastián; Masiero, Joseph; Modjaz, Maryam; Morgan, Adam; Poznanski, Dovi
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Abstract:
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We present the discovery of an absorption-line redshift of z = 2.609 for GRB 090426, establishing
the first firm lower limit to a redshift for a gamma-ray burst (GRB) with an observed
duration of <2 s.With a rest-frame burst duration of T 90z = 0.35 s and a detailed examination
of the peak energy of the event, we suggest that this is likely (at >90 per cent confidence)
a member of the short/hard phenomenological class of GRBs. From analysis of the opticalafterglow
spectrum we find that the burst originated along a very low HI column density
sightline, with NHI < 3.2 × 1019 cm−2. Our GRB090426 afterglow spectrum also appears
to have weaker low-ionization absorption (Si II, CII) than ∼95 per cent of previous afterglow
spectra. Finally, we also report the discovery of a blue, very luminous, star-forming putative
host galaxy (∼2L∗) at a small angular offset from the location of the optical afterglow. We
consider the implications of this unique GRB in the context of burst duration classification
and our understanding of GRB progenitor scenarios. |