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

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.

Propagation of a Short GRB Jet in the Ejecta: Jet Launching Delay Time, Jet Structure, and GW170817/GRB 170817A

“…However, such a simple model fails to explain the prompt emission data of GRB 170817A (e.g., Granot et al 2017;Gottlieb et al 2018;Meng et al 2018). The brightening of the late-time X-ray/optical/radio afterglow hints the existence of a significant energy injection, which was interpreted as either lateral injection from a structured jet (Kathirgamaraju et al 2018;Lazzati et al 2018;Lamb & Kobayashi 2018;Lyman et al 2018;Xie et al 2018;Ghirlanda et al 2019;Troja et al 2018;Piro et al 2019;Xiao et al 2017) or radial injection from a stratified cocoon (Kasliwal et al 2017;Mooley et al 2018b;Nakar & Piran 2017;Margutti et al 2018). The detection of "superluminal" motion in the radio afterglow (Mooley et al 2018a;Ghirlanda et al 2019) ruled out the later scenario and reinforced the structured jet picture.…”

Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail

“…When jets plow through dense environments they deposit a fraction of their energy in a hot cocoon (Ramirez-Ruiz et al 2002;Pe'er et al 2006), which may occur in binary neutron star mergers as the ultrarelativistic jet that powers the SGRB pushes through material ejected just before merger (Lazzati et al 2017). GRB 170817A has odd behavior in both prompt and afterglow emission, the origin of which is a matter of some debate (Abbott et al 2017a;Alexander et al 2017Alexander et al , 2018Bromberg et al 2017;Haggard et al 2017;Kasliwal et al 2017;Margutti et al 2017;Mooley et al 2018aMooley et al , 2018bTroja et al 2017Troja et al , 2018aGottlieb et al 2018;Lazzati et al 2018;Lyman et al 2018;Nynka et al 2018;Ruan et al 2018;Veres et al 2018). Possible interpretations include a structured ultrarelativistic jet (e.g., Alexander et al 2018), a jet and cocoon together (e.g., Abbott et al 2017a), or a cocoon model (e.g., Kasliwal et al 2017) where the shock breakout produces the harder peak.…”

Continued Brightening of the Afterglow of GW170817/GRB 170817A as Being Due to a Delayed Energy Injection

“…Ruan et al (2018) further found that the radio emission brightened at approximately the same rate as that of the X-ray emission, which indicates that they share a common origin (see also Mooley et al 2018). Later, Lyman et al (2018) reported that the late-time optical afterglow of GRB 170817A was continuously brightening until 110days after the merger.…”

“…The fall-back accretion process was first discussed by Colgate (1971). It is originally assumed to be Kim et al (2017), Mooley et al (2018), and Margutti et al (2018). The X-ray data points are taken from Ruan et al (2018), Lazzati et al (2017c), andD'Avanzo et al (2018).…”

“…It has been proposed that the structured jet model or the cocoon model could match the late-time behavior of GRB 170817A (Gottlieb et al 2017;Granot et al 2017;Lamb & Kobayashi 2017;Lazzati et al 2017c;Wang & Huang 2017;Lyman et al 2018;Mooley et al 2018;. A structured outflow with a highly relativistic inner core launched by the merger remnant may interpret the rising afterglow emission Perna et al 2003;Lazzati et al 2004;Starling et al 2005;Rezzolla et al 2011).…”