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The optical afterglow of GW170817: An off-axis structured jet and deep constraints on a globular cluster origin

The optical afterglow of GW170817: An off-axis structured jet and deep constraints on a globular cluster origin
The optical afterglow of GW170817: An off-axis structured jet and deep constraints on a globular cluster origin

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 , 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 gamma-ray bursts 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 (GC) of L ≲ 6.7 × 103 L o, or M ≲ 1.3 × 104 M o, at least 4 standard deviations below the peak of the GC mass function of the host galaxy, NGC 4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a GC. As highlighted here, HST (and soon the James Webb Space Telescope) enables critical observations of the optical emission from neutron star merger jets and outflows.

2041-8205
Fong, W.
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Blanchard, P. K.
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Alexander, K. D.
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Strader, J.
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Margutti, R.
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Hajela, A.
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Villar, V. A.
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Wu, Y.
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Berger, E.
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Chornock, R.
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Giannios, D.
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Guidorzi, C.
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Kathirgamaraju, A.
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Laskar, T.
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Macfadyen, A.
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Metzger, B. D.
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Nicholl, M.
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Paterson, K.
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Sand, D. J.
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Sironi, L.
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Williams, P. K.G.
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Xie, X.
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Zrake, J.
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Fong, W.
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Blanchard, P. K.
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Alexander, K. D.
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Strader, J.
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Margutti, R.
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Hajela, A.
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Villar, V. A.
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Wu, Y.
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Ye, C. S.
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Berger, E.
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Chornock, R.
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Coppejans, D.
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Cowperthwaite, P. S.
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Eftekhari, T.
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Giannios, D.
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Guidorzi, C.
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Kathirgamaraju, A.
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Laskar, T.
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Macfadyen, A.
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Metzger, B. D.
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Nicholl, M.
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Paterson, K.
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Terreran, G.
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Sand, D. J.
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Sironi, L.
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Williams, P. K.G.
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Xie, X.
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Zrake, J.
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Fong, W., Blanchard, P. K., Alexander, K. D., Strader, J., Margutti, R., Hajela, A., Villar, V. A., Wu, Y., Ye, C. S., Berger, E., Chornock, R., Coppejans, D., Cowperthwaite, P. S., Eftekhari, T., Giannios, D., Guidorzi, C., Kathirgamaraju, A., Laskar, T., Macfadyen, A., Metzger, B. D., Nicholl, M., Paterson, K., Terreran, G., Sand, D. J., Sironi, L., Williams, P. K.G., Xie, X. and Zrake, J. (2019) The optical afterglow of GW170817: An off-axis structured jet and deep constraints on a globular cluster origin. Astrophysical Journal Letters, 883 (1), [L1]. (doi:10.3847/2041-8213/ab3d9e).

Record type: Article

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 , 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 gamma-ray bursts 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 (GC) of L ≲ 6.7 × 103 L o, or M ≲ 1.3 × 104 M o, at least 4 standard deviations below the peak of the GC mass function of the host galaxy, NGC 4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a GC. As highlighted here, HST (and soon the James Webb Space Telescope) enables critical observations of the optical emission from neutron star merger jets and outflows.

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The Optical Afterglow of GW170817_An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin - Accepted Manuscript
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Accepted/In Press date: 20 August 2019
e-pub ahead of print date: 17 September 2019

Identifiers

Local EPrints ID: 435320
URI: http://eprints.soton.ac.uk/id/eprint/435320
ISSN: 2041-8205
PURE UUID: a44d361c-5a49-445a-ab30-815d111128c8
ORCID for X. Xie: ORCID iD orcid.org/0000-0002-2798-6880

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Date deposited: 30 Oct 2019 17:30
Last modified: 18 Mar 2024 05:25

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Contributors

Author: W. Fong
Author: P. K. Blanchard
Author: K. D. Alexander
Author: J. Strader
Author: R. Margutti
Author: A. Hajela
Author: V. A. Villar
Author: Y. Wu
Author: C. S. Ye
Author: E. Berger
Author: R. Chornock
Author: D. Coppejans
Author: P. S. Cowperthwaite
Author: T. Eftekhari
Author: D. Giannios
Author: C. Guidorzi
Author: A. Kathirgamaraju
Author: T. Laskar
Author: A. Macfadyen
Author: B. D. Metzger
Author: M. Nicholl
Author: K. Paterson
Author: G. Terreran
Author: D. J. Sand
Author: L. Sironi
Author: P. K.G. Williams
Author: X. Xie ORCID iD
Author: J. Zrake

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