Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers

Gravitational-wave-detected neutron star mergers provide an opportunity to investigate short gamma-ray burst (GRB) jet afterglows without the GRB trigger. Here we show that the postpeak afterglow decline can distinguish between an initially ultrarelativistic jet viewed off-axis and a mildly relativistic wide-angle outflow. Post-peak the afterglow flux will decline as Fν ∝ t −α. The steepest decline for a jet afterglow is α > 3p/4 or > (3p + 1)/4, for an observation frequency below and above the cooling frequency, respectively, where p is the power-law index of the electron energy distribution. The steepest decline for a mildly relativistic outflow, with initial Lorentz factor 0 2, is α (15p − 19)/10 or α (15p − 18)/10, in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index α > 1.5, then we are observing the core of an initially ultrarelativistic jet viewed off the central axis, while a decline with α 1.4 after ∼5–10 peak times indicates that a wide-angled and initially 0 2 outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of α ≈ 1. So far, two post-peak X-ray data points at 160 and 260 d suggest a decline consistent with an off-axis jet afterglow. Follow-up observations over the next 1–2 yr will test this model

GRB jet structure and the jet break

We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, θc. We fit model and observed afterglow light curves with a smoothly-broken power-law function with a free-parameter κ that describes the sharpness of the break. We find that the jet break is sharper (κ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of 0.1 ≲ κ ≲ 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 ≲ κ ≲ 2.2 when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within θc, whereas for smooth-edged jets, κ is more strongly dependent on the size of θc. Using a sample of 20 GRBs, we find 9 candidate smooth-edged jet structures and 8 candidate sharp-edged jet structures, while the remaining 3 are consistent with either. The shape of the jet break, as measured by the sharpness parameter κ, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time

Heavy element production in a compact object merger observed by JWST

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs)1, sources of high-frequency gravitational waves (GW)2 and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process)3. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers4–6, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW1708177–12. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.</p