The afterglow of GRB 170817A/GW 170817 was very unusual, slowly rising as F ν ∝ t 0.8 obs ν −0.6 , peaking at t obs,pk ∼ 150 days, and sharply decaying as ∼ t −2.2 obs . VLBI observations revealed an unresolved radio afterglow image whose flux centroid apparently moved superluminally with v app ≈ 4c between 75 and 230 days, clearly indicating that the afterglow was dominated by a relativistic jet's compact core. Different jet angular structures successfully explained the afterglow lightcurves: Gaussian and steep power-law profiles with narrow core angles θ c 5 • and significantly larger viewing angles θ obs /θ c ∼ 3 − 5. However, a top-hat jet (conical with sharp edges at θ = θ 0 ) was ruled out since it appeared to produce an early flux rise much steeper (∝ t a obs with a 3) than observed. Using 2D relativistic hydrodynamic simulations of an initially top-hat jet we show that the initial steep flux rise is an artifact caused by the simulation's finite start time, t 0 , missing its flux contributions from t < t 0 and sometimes "compensated" using an analytic top-hat jet. While an initially top-hat jet is not very physical, such simulations are particularly useful at t obs t obs,pk when the afterglow emission is dominated by the jet's core and becomes insensitive to its exact initial angular profile if it drops off sharply outside of the core. We demonstrate that an initially top-hat jet fits GW 170817/GRB 170817A's afterglow lightcurves and flux centroid motion at t obs t obs,pk , for θ obs /θ 0 ≈ 3 and may also fit the earlier lightcurves for Γ 0 = Γ(t 0 ) 10 2.5 . We analytically express the degeneracies between the model parameters, and find a minimal jet energy of E min ≈ 5.3 × 10 48 erg and circum-burst medium density of n min ≈ 5.3 × 10 −6 cm −3 .1 Here νm is the synchrotron frequency of minimal energy electrons and νc of electrons that cool on the dynamical time (Sari et al. 1998).

The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray burst arising from a neutron star merger, has challenged our understanding of the angular structure of relativistic jets. Studies of the jet propagation usually assume that the jet is ejected from the central engine with a top-hat structure and its final structure, which determines the observed light curve and spectra, is primarily regulated by the interaction with the nearby environment. However, jets are expected to be produced with a structure that is more complex than a simple top-hat, as shown by global accretion simulations. We present numerical simulations of short GRBs launched with a wide range of initial structures, durations and luminosities. We follow the jet interaction with the merger remnant wind and compute its final structure at distances ≳ 1011 cm from the central engine. We show that the final jet structure, as well as the resulting afterglow emission, depend strongly on the initial structure of the jet, its luminosity and duration. While the initial structure at the jet is preserved for long-lasting SGRBs, it is strongly modified for jets barely making their way through the wind. This illustrates the importance of combining the results of global simulations with propagation studies in order to better predict the expected afterglow signatures from neutron star mergers. Structured jets provide a reasonable description of the GRB 170817A afterglow emission with an off-axis angle θobs ≈ 22.5○.

After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray burst (GRB), several studies tried to understand the structure of GRB jets. The initial jet structure (directly produced by the central engine) can be partially preserved, or can be completely modified by the interaction with the environment. In this study, we perform three-dimensional, special relativistic hydrodynamics simulations of long GRB jets evolving through a massive progenitor star. Different jet scenarios were considered: Top-hat, Gaussian jets dominated by pressure or by kinetic energy, as well as a model of a supernova (SN) plus a jet both propagating through the progenitor. We found that, while propagating inside the progenitor star, jets with different initial structures are nearly indistinguishable. Kinetic dominated jets are faster and more collimated than pressure dominated jets. The dynamics of jets inside the progenitor star strongly depends on the presence of an associated SN, which can substantially decelerate the jet propagation. We show that the initial structure of GRB jets is preserved, or not, mainly depending on the jet collimation. The initial structure is preserved in uncollimated jets, i.e. jets which move through low density environments. Meanwhile, jets which move through dense environments are shaped by the interaction with the medium and remain collimated.

Gamma-ray bursts (GRBs) are produced during the propagation of ultra-relativistic jets. It is challenging to study the jet close to the central source, due to the high opacity of the medium. In this paper, we present numerical simulations of relativistic jets propagating through a massive, stripped envelope star associated to long GRBs, breaking out of the star and accelerating into the circumstellar medium. We compute the gravitational wave (GW) signal resulting from the propagation of the jet through the star and the circumstellar medium. We show that key parameters of the jet propagation can be directly determined by the GW signal. The signal presents a first peak corresponding to the jet duration and a second peak which corresponds to the break-out time for an observer located close to the jet axis (which in turn depends on the stellar size), or to much larger times (corresponding to the end of the acceleration phase) for off-axis observers. We also show that the slope of the GW signal before and around the first peak tracks the jet luminosity history and the structure of the progenitor star. The amplitude of the GW signal is h+D ∼ hundreds to several thousands cm. Although this signal, for extragalactic sources, is outside the range of detectability of current GW detectors, it can be detected by future instruments as BBO, DECIGO and ALIA. Our results illustrate that future detections of GW associated to GRB jets may represent a revolution in our understanding of this phenomenon.

After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray burst (GRB), several studies tried to understand the structure of GRB jets. The initial jet structure (directly produced by the central engine) can be partially preserved, or can be completely modified by the interaction with the environment. In this study, we perform three-dimensional, special relativistic hydrodynamics simulations of long GRB jets evolving through a massive progenitor star. Different jet scenarios were considered: Top-hat, Gaussian jets dominated by pressure or by kinetic energy, as well as a model of a supernova (SN) plus a jet both propagating through the progenitor. We found that, while propagating inside the progenitor star, jets with different initial structures are nearly indistinguishable. Kinetic dominated jets are faster and more collimated than pressure dominated jets. The dynamics of jets inside the progenitor star strongly depends on the presence of an associated SN, which can strongly decelerate the jet propagation. We show that the initial structure of GRB jets is preserved, or not, mainly depending on the jet duration (𝑡 𝑗 ) and on the jet break-out time (𝑡 bo ) from the dense environment. The initial structure is preserved in long-lasting jets (𝑡 𝑗 𝑡 bo ), while the interaction with the medium shapes short duration jets (𝑡 𝑗 𝑡 bo ). Future observations of a large sample of off-axis GRB will constrain the density stratification of the environment, the initial structure of the jet, and the physics of the central engine itself.