A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817
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 launched during the merger, directed away from our line of sight. The presence of such a jet is predicted from models that posit neutron-star mergers as the central engines that drive short hard γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of an off-axis jet afterglow. Although we cannot rule out the existence of a jet pointing elsewhere, the observed γ-rays could not have originated from such a jet. Instead, the radio data require a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material dynamically ejected during the merger or a cocoon of material that breaks out when a jet transfers its energy to the dynamical ejecta. The cocoon model explains the radio light curve of GW170817 as well as the γ-rays and X-rays (possibly also ultraviolet and optical emission), and is therefore the model most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a heretofore unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.
© 2017 Macmillan Publishers Limited, part of Springer Nature. Received: 28 November 2017; Accepted: 11 December 2017; Published online: 20 December 2017. We acknowledge the support and dedication of the staff of the National Radio Astronomy Observatory and particularly thank the VLA Director, M. McKinnon, as well as A. Mioduszewski and H. Medlin, for making the VLA campaign possible. We thank Britt Griswold (NASA/GSFC) for beautiful graphic arts. SK thanks M. Shull for discussions. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the GMRT staff for scheduling our observations. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. KM's research is supported by the Hintze Centre for Astrophysical Surveys which is funded through the Hintze Family Charitable Foundation. EN acknowledges the support of an ERC starting grant (GRB/SN) and an ISF grant (1277/13). GH acknowledges the support of NSF award AST-1654815.AC acknowledges support from the National Science Foundation CAREER award #1455090 titled 'CAREER: Radio and gravitational-wave emission from the largest explosions since the Big Bang'. AH acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. TM acknowledges the support of the Australian Research Council through grant FT150100099. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. DK was supported by NSF grant AST-1412421. MK's work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE Grant No 1545949. This work is part of the research program Innovational Research Incentives Scheme (Vernieuwingsimpuls), which is financed by the Netherlands Organization for Scientific Research through the NWO VIDI Grant No. 639.042.612-Nissanke and NWO TOP Grant No. 62002444--Nissanke. PC acknowledges support from the Department of Science and Technology via SwarnaJayanti Fellowship awards (DST/SJF/PSA-01/2014-15). TP acknowledges the support of Advanced ERC grant TReX. VB acknowledges the support of the Science and Engineering Research Board, Department of Science and Technology, India, for the GROWTH-India project. Author Contributions: KM, EN, KH, GH and DF wrote the paper. AC compiled the references. AC and AH compiled the methods section. DD and KD compiled the radio measurements table. KM managed the VLA observing program and processed all the VLA data. SM, AD and SB helped plan the VLA observations. EN, KH, DK and KM prepared the figures. TM planned and managed ATCA observations and data analysis and contributed to the manuscript text. DK helped propose for and plan the ATCA observations and contributed to the manuscript text. EL, DD, CL and KB helped with ATCA observations and data reduction. KD planned and managed GMRT observations and contributed to manuscript text. KM and PC processed the GMRT data. VB helped in the GMRT observations. OG and EN provided the cocoon simulation. KH provided the spherical ejecta model. SN did the GW and cocoon rates analysis. SK, TP, MK and LS provided text for the paper. All coauthors discussed the results and provided comments on the manuscript. Data availability: All relevant data are available from the corresponding author on request. Data presented in Figure 1 are included in Extended Data Table 1. Code availability: The codes used for generating the synthetic radio light curves are currently being readied for public release (publication in preparation). Radio data processing software: CASA, MIRIAD, DIFMAP.
Accepted Version - 1711.11573.pdf