Abstract
We report the detection of the radio afterglow of a long gamma-ray burst (GRB) 111005A at 5-345 GHz, including very long baseline interferometry observations with a positional error of 0.2 mas. The afterglow position is coincident with the disc of a galaxy ESO 58049 at z = 0.01326 (∼1″ from its centre), which makes GRB 111005A the second-closest GRB known to date, after GRB 980425. The radio afterglow of GRB 111005A was an order of magnitude less luminous than those of local low-luminosity GRBs, and obviously less luminous than those of cosmological GRBs. The radio flux was approximately constant and then experienced an unusually rapid decay a month after the GRB explosion. Similarly to only two other GRBs, we did not find the associated supernovae (SNe), despite deep near- and mid-infrared observations 1-9 days after the GRB explosion, reaching ∼20 times fainter than other SNe associated with GRBs. Moreover, we measured a twice-solar metallicity for the GRB location. The low y-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a rare class of GRB that is different from typical core-collapse events. We modelled the spectral energy distribution of the GRB 111005A host finding that it is a moderately star-forming dwarf galaxy, similar to the host of GRB 980425. The existence of two local GRBs in such galaxies is still consistent with the hypothesis that the GRB rate is proportional to the cosmic star formation rate (SFR) density, but suggests that the GRB rate is biased towards low SFRs. Using the far-infrared detection of ESO 580-49, we conclude that the hosts of both GRBs 111005A and 980425 exhibit lower dust content than what would be expected from their stellar masses and optical colors.
Highlights
Long gamma ray-burst (GRBs) have been shown to be collapses of very massive stars (e.g. Hjorth et al 2003; Stanek et al 2003; see a review in Hjorth & Bloom 2012), and because of very short main-sequence lifetimes of such stars, gamma-ray burst (GRB) are expected to trace galaxies with on-going starformation
The low γ-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a rare class of GRB that is different from typical core-collapse events
Using the 5–345 GHz observations of the afterglow of GRB 111005A, we found that it is located in an edge-on disc galaxy at z = 0.01326, which makes it the second closest GRB known to date
Summary
Long (duration >2 s) gamma ray-burst (GRBs) have been shown to be collapses of very massive stars (e.g. Hjorth et al 2003; Stanek et al 2003; see a review in Hjorth & Bloom 2012), and because of very short main-sequence lifetimes of such stars, GRBs are expected to trace galaxies with on-going starformation (but see Rossi et al 2014). This could potentially be used as a tool to study cosmic star formation rate (SFR) density, but requires prior understanding of GRBs and their host galaxies. Low-z GRBs are promising candidates for the detection of non-electromagnetic signals such as gravitational waves and neutrinos
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