Abstract
We present new Jansky Very Large Array observations of five pre-Swift gamma-ray bursts for which an ultraluminous (SFR > 100 M_sun/yr) dusty host galaxy had previously been inferred from radio or submillimetre observations taken within a few years after the burst. In four of the five cases we no longer detect any source at the host location to limits much fainter than the original observations, ruling out the existence of an ultraluminous galaxy hosting any of these GRBs. We continue to detect a source at the position of GRB 980703, but it is much fainter than it was a decade ago and the inferred radio star-formation rate (~80 M_sun) is relatively modest. The radio flattening at 200-1000 days observed in the light curve of this GRB may have been caused by a decelerating counterjet oriented 180 degrees away from the viewer, although an unjetted wind model can also explain the data. Our results eliminate all well-established pre-Swift ULIRG hosts, and all cases for which an unobscured GRB was found in a galaxy dominated by heavily-obscured star-formation. When GRBs do occur in ULIRGs the afterglow is almost always observed to be heavily obscured, consistent with the large dust opacities and high dust covering fractions characteristic of these systems.
Highlights
Long gamma-ray bursts (GRBs) are produced by the explosion of massive, short-lived stars at cosmological distances (Hjorth & Bloom 2012)
These include submillimetre galaxies (SMGs; galaxies at cosmological redshift detected at 850 μm with single-dish telescopes), ultraluminous infrared galaxies (ULIRGs; galaxies with infrared luminosity exceeding >1012 L ), and similar systems containing extensive dust-obscured star formation
Standard IR-based star formation rate indicators (e.g. Calzetti 2013) imply that a star formation rate of ∼100 M yr−1 is required to power a typical threshold ULIRG with LIR = 1012 L, so our observations strongly suggest that none of these galaxies are ULIRGs
Summary
Long gamma-ray bursts (GRBs) are produced by the explosion of massive, short-lived stars at cosmological distances (Hjorth & Bloom 2012). One type of galaxy we may expect to frequently observe GRBs originating from is the broad class of luminous, dusty star-forming galaxies (DSGs) These include submillimetre galaxies (SMGs; galaxies at cosmological redshift detected at 850 μm with single-dish telescopes), ultraluminous infrared galaxies (ULIRGs; galaxies with infrared luminosity exceeding >1012 L ), and similar systems containing extensive dust-obscured star formation. Deep radio and Herschel surveys have produced a few secure examples of DSGs with star formation rates >100– 300 M hosting GRBs (Perley & Perley 2013; Hunt et al 2014; Schady et al 2014; Perley et al 2015) None of these would have been detected to the shallower limits of pre-Swift observations – and the bursts hosting them were heavily obscured in almost all cases, even though both obscured and unobscured GRBs were searched.
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