Illuminating gravitational waves: A concordant picture of photons from a neutron star merger

Abstract

Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.

Swift and NuSTAR observations of GW170817: Detection of a blue kilonova

“…It confirms that binary neutron star mergers can generate short gamma-ray transients (51), though the connection to classical short GRBs remains unclear. Furthermore, GW 170817 provides robust evidence that r-process nucleosynthesis occurs in the aftermath of a binary neutron star merger (10). While a kilonova detection following a short GRB has been previously reported (52,53), our multi-wavelength dataset has allowed us confront kilonova models with UV and X-ray observations.…”

“…This requires an alternative explanation for the observed γ-ray emission, such as a (typical) short GRB viewed (slightly) off-axis, or the emission from a cocoon formed by the interaction of a jet with the merger ejecta (26)(27)(28). We return to this issue below in the context of late-time (∆t >∼ 10 d) X-ray emission (see also (10) and (9)). …”

“…For the median values of short GRB afterglow energy, E AG = 2 × 10 51 erg, circumburst density, n 0 = 5 × 10 −3 cm −3 , and jet opening angle, θ jet = 0.2 radian This inferred orientation is entirely consistent with the results of our analysis of the early UV emission. However, it is difficult to simultaneously explain the observed gamma-ray emission in this scenario, as it would require a viewing angle only slightly outside the jet edge (10). Either the observed GRB 170817A is powered by a source distinct from this jet, or we are forced to disfavor an orphan afterglow model for the late-time X-ray emission.…”

“…The latest NuSTAR datapoint disfavors energetic cocoon models, particularly those at high density. But lower energy or density models can fit all the X-ray data, whilst simultaneously accounting for the gamma-ray emission (10).…”

Afterglows and Kilonovae Associated with Nearby Low-luminosity Short-duration Gamma-Ray Bursts: Application to GW170817/GRB 170817A

“…Yamazaki et al 2002Yamazaki et al , 2003. The fact that GRB170817A has a typical peak energy E p would not conflict with these models, since the prompt emission mechanism is unknown and the observed gamma-rays could either arise from the emission of the jet scattered to a wide angle (Kisaka et al 2017) or just from the emission produced as the cocoon breaks out of the ejecta (Gottlieb et al 2017b;Kasliwal et al 2017).…”