A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817
Journal article, 2018
GW170817 was the first gravitational-wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis). The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission), it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.
Author
K. Mooley
National Radio Astronomy Observatory Socorro
California Institute of Technology (Caltech)
E. Nakar
Tel Aviv University
K. Hotokezaka
Princeton University
G. Hallinan
California Institute of Technology (Caltech)
A. Corsi
Texas Tech University at Lubbock
D. A. Frail
National Radio Astronomy Observatory Socorro
A. Horesh
The Hebrew University Of Jerusalem
T. Murphy
ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)
The University of Sydney
E. Lenc
The University of Sydney
ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)
D. L. Kaplan
University of Wisconsin Milwaukee
K. De
California Institute of Technology (Caltech)
D. Dobie
The University of Sydney
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)
P. Chandra
National Centre for Radio Astrophysics India
Stockholm University
A. Deller
ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)
Swinburne University of Technology
O. Gottlieb
Tel Aviv University
M. Kasliwal
California Institute of Technology (Caltech)
S. R. Kulkarni
California Institute of Technology (Caltech)
S. T. Myers
National Radio Astronomy Observatory Socorro
S. Nissanke
Radboud University
T. Piran
The Hebrew University Of Jerusalem
C. Lynch
ARC Centre of Excellence for All-sky Astrophysics (CAASTRO)
The University of Sydney
V. Bhalerao
Indian Institute of Technology
Stephen Bourke
Chalmers, Space, Earth and Environment, Onsala Space Observatory
K. W. Bannister
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
L. P. Singer
NASA Goddard Space Flight Center
Nature
0028-0836 (ISSN) 1476-4687 (eISSN)
Vol. 554 7691 207-210Subject Categories
Accelerator Physics and Instrumentation
Subatomic Physics
Astronomy, Astrophysics and Cosmology
DOI
10.1038/nature25452