The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VIII. A Comparison to Cosmological Short-duration Gamma-ray Bursts

TL;DR

This paper compares the electromagnetic counterpart of GW170817 with short gamma-ray bursts, revealing similarities in jet energies and differences mainly due to viewing angles, and discusses implications for kilonovae and host galaxy properties.

Contribution

It provides a detailed comparison of GW170817's electromagnetic signals with short GRBs, highlighting the role of viewing angles and host galaxy characteristics in understanding neutron star mergers.

Findings

GW170817's luminosity is much lower than on-axis short GRBs.

Jet energies and densities are similar, suggesting viewing angle effects dominate.

GW170817's host galaxy is older and less star-forming than typical short GRB hosts.

Abstract

We present a comprehensive comparison of the properties of the radio through X-ray counterpart of GW170817 and the properties of short-duration gamma-ray bursts (GRBs). For this effort, we utilize a sample of 36 short GRBs spanning a redshift range of $z \approx 0.12-2.6$ discovered over 2004-2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is $\approx 3000$ times less than the median value of on-axis short GRB X-ray afterglows, and $\gtrsim10^{4}$ times less than that for detected short GRB radio afterglows. Moreover, the allowed jet energies and particle densities inferred from the radio and X-ray counterparts to GW170817 and on-axis short GRB afterglows are remarkably similar, suggesting that viewing angle effects are the dominant, and perhaps only, difference in their observed radio and X-ray behavior. From comparison to previous claimed kilonovae following short GRBs, we find that the optical and near-IR counterpart to GW170817 is comparatively under-luminous by a factor of $\approx 3-5$, indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on $\lesssim 1$ day timescales also rules out a "blue" kilonova of comparable optical isotropic-equivalent luminosity in one previous short GRB. Finally, we investigate the host galaxy of GW170817, NGC4993, in the context of short GRB host galaxy stellar population properties. We find that NGC4993 is superlative in terms of its large luminosity, old stellar population age, and low star formation rate compared to previous short GRB hosts. Additional events within the Advanced LIGO/VIRGO volume will be crucial in delineating the properties of the host galaxies of NS-NS mergers, and connecting them to their cosmological counterparts.