The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

Abstract

We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at ≈584 days post-merger, which provide a robust optical template. The light curve spans ≈ 110 − 362 days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of β = −0.583 ± 0.013, demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster of L 6.7 × 10 3 L , or M 1.3 × 10 4 M , at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy, NGC4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster. As highlighted here, HST (and soon JWST) enables critical observations of the optical emission from neutron star merger jets and outflows.

Searching for electromagnetic counterparts to gravitational-wave merger events with the prototype Gravitational-Wave Optical Transient Observer (GOTO-4)

“…KNe and sGRBs were also confirmed to be linked to BNS mergers by the detections of GRB 170817A (Abbott et al 2017b;Goldstein et al 2017;Hallinan et al 2017;Margutti et al 2017;Savchenko et al 2017;Troja et al 2017;Margutti et al 2018;Mooley et al 2018;Troja et al 2018b;D'Avanzo et al 2018;Lyman et al 2018) and the associated kilonova AT2017gfo (Blanchard et al 2017;Chornock et al 2017;Coulter et al 2017;Covino et al 2017;Cowperthwaite et al 2017;Drout et al 2017;Evans et al 2017;Hjorth et al 2017;Levan et al 2017;Nicholl et al 2017;Pian et al 2017;Smartt et al 2017;Soares-Santos et al 2017;Tanvir et al 2017;Villar et al 2017) alongside GW170817 (Abbott et al 2017a). In this instance, the sGRB afterglow was likely viewed somewhat away from the jet axis (Abbott et al 2017a;Haggard et al 2017;Kim et al 2017;Lazzati et al 2017;Lamb & Kobayashi 2018;Mandel 2018;Fong et al 2019;Lamb et al 2019a;Wu & MacFadyen 2019).…”

Prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo and KAGRA

“…An X-ray signal (Troja et al 2017;Margutti et al 2017;Haggard et al 2017;Ruan et al 2018;Pooley et al 2017) and a radio signal (Hallinan et al 2017;Alexander et al 2017;Mooley et al 2018) were discovered at the position of the optical transient after $ 9 days and $ 16 days, respectively. A slow multi-wavelength flux-rise of the non-thermal emission was observed until $ 150 days (Lyman et al 2018;Margutti et al 2018;Troja et al 2018) before entering a flattening-decaying phase (D'Avanzo et al 2018;Dobie et al 2018;Alexander et al 2018;Hajela et al 2019;Fong et al 2019). Very Long Baseline Interferometry observations enabled measurement of the superluminal proper motion of the radio counterpart (Mooley et al 2018) and constrained the apparent size of the source (Ghirlanda et al 2019), proving that a relativistic and narrowlycollimated jet successfully emerged from the neutron star merger.…”

“… 2019 ; Fong et al. 2019 ). Very Long Baseline Interferometry observations enabled measurement of the superluminal proper motion of the radio counterpart (Mooley et al.…”

Bayesian inference for compact binary coalescences with bilby: validation and application to the first LIGO–Virgo gravitational-wave transient catalogue

“…The inferred source parameters can inform our understanding of binary stellar evolution (Stevenson et al 2015;Abbott et al 2016d;Zevin et al 2017;Abbott et al 2017h; Barrett et al 2018;Belczynski et al 2018;Bavera et al 2020), the equation of state of neutron-star matter (Abbott et al 2018c;Most et al 2018;Essick et al 2020;Abbott et al 2020b), and the nature of gravity (Yunes & Siemens 2013;Abbott et al 2016b;Yunes et al 2016;Abbott et al 2019i;Isi et al 2019). Multimessenger observations of gravitational and electromagnetic radiation (Abbott et al 2017e) can give an even richer understanding, enabling measurements of cosmological parameters (Abbott et al 2017d(Abbott et al , 2019dCantiello et al 2018;Hotokezaka et al 2019;Dhawan et al 2020;Chen et al 2018), insights into the structures of gamma-ray bursts (Abbott et al 2017f;Mooley et al 2018;Margutti et al 2018;Fong et al 2019;Biscoveanu et al 2020b), and identifying the origins of heavy elements (Abbott et al 2017g;Chornock et al 2017;Tanvir et al 2017;Kasliwal et al 2019;Watson et al 2019). However, electromagnetic emission can fade isobel.romero-shaw@monash.edu rapidly, necessitating rapid localization of the gravitationalwave source (Abbott et al 2018b).…”