Jet or no jet? The continued intrigue of GW170817

GW170817, the first multi-messenger gravitational wave event, became an instant arxiv-chart topper, but it was no one-hit wonder. Following the furore surrounding its discovery in August last year, there has been a resurgence in interest for GW170817. Conflicting interpretations of its late-time emission have called into doubt the link between GW170817 and cosmological gamma-ray bursts and raised questions about the aftermath of neutron star mergers.

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The paper in Nature Astronomy is here:

The discovery of GW170817 is the kind of generational breakthrough in astrophysics that will define a new field of research for decades to come. A single event that offers the source of r-process element production, the progenitor systems of short gamma-ray bursts, an independent measure of the cosmology of the Universe, and limits on the neutron star equation of state, naturally causes quite a splash. It was extremely exciting to be caught in the midst of its discovery and the intensive follow up that ensued. That is, until it disappeared to most telescopes behind the Sun about a month later.

Although radio telescopes continued monitoring the source, as a primarily optical observer it was frustrating waiting for it to reappear. As soon as we were allowed, we slewed the Hubble Space Telescope (HST) to observe it after it re-emerged. We weren't expecting much - the fabled kilonova observed at early times was expected to be far beyond the reach of even HST. However, after a bit of squinting (and somewhat more quantitative analysis) it looked like we had something.

Combined with reports of radio and X-ray results, it looked like we had found the optical counterpart associated with the emission from relativistic material expelled during the merger, known as the gamma-ray burst (GRB) afterglow.

A zoom in on the galaxy NGC 4993 showing the optical detection of GW170817 by HST at 110 days after the merger (indicated by arrow) - squinting may be necessary.

The afterglow was proving a struggle to fit with the classical jet scenario inferred for cosmological GRBs. In this scenario a highly collimated, highly relativistic jet is ejected along the polar axis of the merger, which are accompanied by an intensively luminous GRB when the axis is aligned with the Earth. Two things made GW170817 stick out from this picture: 1) The GRB we saw associated with it was extremely weak, 2) the gravitational wave data suggested we were viewing it significantly away from the polar axis.

As such the 'cocoon' model came to the fore. Rather than a beamed highly relativistic jet punching its way out, energy is deposited into the expanding ejecta surrounding the merger, accelerating a large, quasi-isotropic portion of the ejecta to mildly relativistic speeds. This then could explain the weak GRB and the fact we have seen it, and the afterglow, significantly 'off-axis'. This scenario however, would need us to reconsider GW170817 in the light of other short GRBs, since it infers a very different emission mechanism. Taking the Occam's razor approach, we decided to revisit the data in light of the jet scenario.

Rather than considering just a single pencil-like jet escaping from the system, we fuzzed the edges somewhat and allowed the jet to expand radially as a narrow core plus wider angle Gaussian component - a so-called 'structured jet'. We found this model fit the data equally well. Up to this point we were championing the structured jet scenario through its harmony with cosmological GRBs, but the data could not distinguish the true model. In the paper we made predictions for how quickly this emission should begin to fade, showing that the structured jet model fades significantly quicker than the cocoon.

Being in the transient business means the Universe changes on time-scales shorter than publication time-scales; subsequent data were arriving that would test our hypothesis concurrently with the peer-review process. Happily enough however, a turn over of the afterglow has now been observed and it indeed appears to be declining at a rate consistent with our prediction for the structured jet. Along with other recent evidence, it appears a successful jet was indeed launched in GW170817.

Observations for the advanced gravitational wave detectors begin again in 2019. We are hoping for many more multi-messenger discoveries, but I have a feeling it will be a while before we see one quite like GW170817 again.

Joe Lyman

Post-doctoral fellow, University of Warwick