Observational follow up of well localized short gamma-ray bursts (SGRBs) has left 20–30 per cent of the population without a coincident host galaxy association to deep optical and NIR limits (≳26 mag). These SGRBs have been classified as observationally hostless due to their lack of strong host associations. It has been argued that these hostless SGRBs could be an indication of the large distances traversed by the binary neutron star system (due to natal kicks) between its formation and its merger (leading to an SGRB). The distances of GRBs from their host galaxies can be indirectly probed by the surrounding circumburst densities. We show that a lower limit on those densities can be obtained from early afterglow light curves. We find that ≲16 per cent of short GRBs in our sample took place at densities ≲10−4 cm−3. These densities represent the expected range of values at distances greater than the host galaxy’s virial radii. We find that out of the five SGRBs in our sample that have been found to be observationally hostless, none are consistent with having occurred beyond the virial radius of their birth galaxies. This implies one of two scenarios. Either these observationally hostless SGRBs occurred outside of the half-light radius of their host galaxy, but well within the galactic halo, or in host galaxies at moderate to high redshifts (z ≳ 2) that were missed by follow-up observations.
Bibliographical noteFunding Information:
The authors thank the referee for their quick response and helpful comments that improved the manuscript. The authors would also like to acknowledge Kenta Hotokezaka, Ehud Nakar, Alexander van der Horst, and Sylvain Guiriec for helpful discussions, and Eleonora Troja for useful comments that improved the manuscript. The authors likewise acknowledge Phillip F. Hopkins for discussions involving the gas density profile of galaxies. The work of BO was supported in part by the National Aeronautics and Space Administration through grants NNX16AB66G, NNX17AB18G, and 80NSSC20K0389. The research of PB was funded by the Gordon and Betty Moore Foundation through Grant GBMF5076. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. The computations were performed on the George Washington University (GWU) Colonial One and Pegasus computer clusters.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
- Gamma-ray bursts
- Neutron star mergers
- Stars: jet