Late Time Afterglow Observations Reveal a Collimated Relativistic Jet in the Ejecta of the Binary Neutron Star Merger GW170817

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multiwavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubt on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multiwavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30° away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.

Figure 1

Light curves of the afterglow of a structured jet calculated at a frequency of 8 GHz (left panel) and 1 keV (right panel) for several observers. Viewing angles are indicated in the legend. The parameters used for these models are ${\epsilon }_e={10}^{-2}$, ${\epsilon }_B={10}^{-3}$, $p_{\mathrm{el}}=2.3$, and $n_{\mathrm{ISM}}={10}^{-4}{\mathrm{cm}}^{-3}$.

Figure 2

The radio, optical, and $x$-ray light curves from a structured jet model are shown in blue (3 GHz), yellow (6 GHz), green ($R$ band), and pink ($x$ rays), respectively. For viewing purposes, all but the 6 GHz curves have been multiplied by a constant (as specified in the legend). Radio and $x$-ray measurements with uncertainties are shown with filled symbols. Additional data at other radio frequencies have been used in the fit but are not shown because they are sparse and would overcrowd the figure. The optical light curve at early time was dominated by the kilonova emission, which is not considered in our modeling. The model shown has ${\chi }^2=69$ with 56 degrees of freedom. The observer lies at an angle of 33° from the jet axis and the fireball propagates in a uniform external medium with number density $n_{\mathrm{ISM}}=4.2\times {10}^{-3}{\mathrm{cm}}^{-3}$. The figure includes the most recent Chandra observation ([59], the $x$-ray datum at 260 days), which is not fit but just overlaid on the best-fit curves from the data at pervious times.

Figure 3

Light curves and their uncertainty areas compared with the measurement of the afterglow of GW170817. The pink line is the best-fit model. The green shaded area is the envelope of all models consistent with the data at the $1\sigma$ level, orange is consistent at the $2\sigma$ level, and brown is consistent at the $3\sigma$ level. Data are displayed with solid blue markers. All models were fit simultaneously to the entire multiwavelength data set. The upper left panel shows the 3 GHz light curve, the upper right panel shows the 6 GHz light curve, the bottom left panel shows the 606 nm light curve, and the bottom right panel shows the 1 keV light curve.

Figure 4

Corner plot showing the degeneracy of the fit parameters for the structured jet model. A strong degeneracy is evident between the interstellar density and the observing angle. The meaning of different colors is the same as in Fig. 3.

Figure 5

Comparison among the best fits 3 GHz light curves for the three model structures discussed: an isotropic fireball (pink), an off-axis, top-hat jet (green), and a structured jet (orange). Data at 3 GHz are shown with solid blue symbols, but the fits were performed on all of the multiwavelength data set. The inset shows the best-fit energy and Lorentz factor profiles of the three models. The vertical arrows show the location of the observer in the top-hat and structured models.

Figure 6

Left panel: pseudocolor density image of the hydrodynamic numerical simulation of a short gamma-ray burst jet from a binary neutron star merger used to compute the afterglow light curves [33]. The propagation of the jet through nonrelativistic ejecta in the vicinity of the merger site causes the emergence of a structured jet, with a light, fast core (the low-density, blue region) and less energetic, slower wings (the orange and green material surrounding the jet). The line of sight to the observer (lying at 33° from the jet axis) is shown with a white arrow. The polar distribution of energy and velocity of the ejecta is shown in the bottom right panel. The top right panel shows the best-fit afterglow model decomposed into radiation coming from the core of the jet (blue), the fast wings of the jet (orange), the material moving along the line of sight (green), and material at large angles (pink and brown, which do not contribute to the observed afterglow emission). The solid black line is the sum of the colored lines.

Figure 7

Broadband spectra of the transient produced by the external shock from a structured jet seen off axis. The afterglow spectrum from the radio to the $x$ rays is shown at two epochs. In blue is the time of the earliest radio and $x$-ray measurements ($\sim 16$ days after the trigger); in orange is the epoch at which the radio light curve peaks, approximately 145 days after the trigger. The displayed data have been taken within a few days of the time at which the spectra were computed (as specified in the legend).