Abstract
Abstract The historic first joint detection of both gravitational-wave and electromagnetic emission from a binary neutron star merger cemented the association between short gamma-ray bursts (SGRBs) and compact object mergers, as well as providing a well-sampled multi-wavelength light curve of a radioactive kilonova (KN) for the first time. Here, we compare the optical and near-infrared light curves of this KN, AT 2017gfo, to the counterparts of a sample of nearby (z < 0.5) SGRBs to characterize their diversity in terms of their brightness distribution. Although at similar epochs AT 2017gfo appears fainter than every SGRB-associated KN claimed so far, we find three bursts (GRBs 050509B, 061201, and 080905A) where, if the reported redshifts are correct, deep upper limits rule out the presence of a KN similar to AT 2017gfo by several magnitudes. Combined with the properties of previously claimed KNe in SGRBs this suggests considerable diversity in the properties of KN drawn from compact object mergers, despite the similar physical conditions that are expected in many NS–NS mergers. We find that observer angle alone is not able to explain this diversity, which is likely a product of the merger type (NS–NS versus NS–BH) and the detailed properties of the binary (mass ratio, spins etc.). Ultimately disentangling these properties should be possible through observations of SGRBs and gravitational-wave sources, providing direct measurements of heavy element enrichment throughout the universe.
Highlights
Short gamma-ray bursts (SGRBs) have long been thought to be the products of the mergers of compact objects (Rosswog et al 2003; Belczynski et al 2006; Nakar 2007)—either binary neutron star (BNS) or neutron star–black hole (NS–BH) systems
Based on these light curves, our sample broadly divides into four main categories: 1. short gamma-ray bursts (SGRBs) with deep limits constraining to an AT 2017gfolike KN
Our analysis reveals a diverse range of KN possibilities, as in some SGRBs we find upper limits for optical/NIR emission several magnitudes deeper than AT 2017gfo, in others there are identified KNe that are brighter, and we find SGRBs with bright afterglows capable of masking KNe that are brighter still
Summary
Short gamma-ray bursts (SGRBs) have long been thought to be the products of the mergers of compact objects (Rosswog et al 2003; Belczynski et al 2006; Nakar 2007)—either binary neutron star (BNS) or neutron star–black hole (NS–BH) systems. A broadband synchrotron afterglow, with emission ranging from X-ray to radio frequencies, is produced as the outflow decelerates in the circumstellar environment (Mészáros & Rees 1993) Such a merger is expected to produce a faint optical/near-IR (NIR) transient known as a “kilonova” (KN, or “macronova”; Li & Paczyński 1998; Rosswog 2005; Metzger et al 2010) as ejected material rich in neutrons forms heavy elements through rapid neutron capture (r-process) nucleosynthesis (Lattimer & Schramm 1974; Eichler et al 1989; Freiburghaus et al 1999) that subsequently decay radioactively. All reported errors are 1σ, and given upper limits are 3σ
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