A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817

K. P. Mooley, E. Nakar, K. Hotokezaka, G. Hallinan, A. Corsi, D. A. Frail, A. Horesh, T. Murphy, E. Lenc, D. L. Kaplan, K. De, D. Dobie, P. Chandra, A. Deller, O. Gottlieb, M. M. Kasliwal, S. R. Kulkarni, S. T. Myers, S. Nissanke, T. PiranC. Lynch, V. Bhalerao, S. Bourke, K. W. Bannister, L. P. Singer

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Abstract

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.

Original languageEnglish
Pages (from-to)207-210
Number of pages4
JournalNature
Volume554
Issue number7691
DOIs
StatePublished - Feb 8 2018

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    Mooley, K. P., Nakar, E., Hotokezaka, K., Hallinan, G., Corsi, A., Frail, D. A., Horesh, A., Murphy, T., Lenc, E., Kaplan, D. L., De, K., Dobie, D., Chandra, P., Deller, A., Gottlieb, O., Kasliwal, M. M., Kulkarni, S. R., Myers, S. T., Nissanke, S., ... Singer, L. P. (2018). A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817. Nature, 554(7691), 207-210. https://doi.org/10.1038/nature25452