- The paper demonstrates that precise VLBI measurements at 207.4 days post-merger constrain the radio source to under 2.5 mas, supporting the structured jet model.
- A joint multi-band analysis, including radio, optical, and X-ray data, reveals an angularly stratified energy profile with a displacement of 2.67 ± 0.3 mas.
- These findings refute the cocoon model and imply that a significant fraction of neutron star mergers produce successful relativistic jets, impacting sGRB studies.
Insights into Structured Jets from Binary Neutron Star Merger GW170817
The paper examines the aftermath of the binary neutron star merger event GW170817, focusing specifically on the debate surrounding the nature of the jet ejected post-merger. Through detailed analysis utilizing Very Long Baseline Interferometry (VLBI) data, the authors conclude that the event produced a structured relativistic jet, effectively excluding the isotropic outflow or cocoon model as a viable possibility.
Observations and Methodology
VLBI observations were conducted 207.4 days after the merger using a comprehensive global network of 32 radio telescopes across diverse continents. The core aspect of the analysis relied on the high spatial resolution provided by VLBI to constrain the apparent size of the radio source associated with the merger. Observations set this upper limit at 2.5 milliarcseconds with 90% confidence. Such measurements provided crucial insight as an isotropic outflow would predict an angular size exceeding this constraint. The authors employed a joint analysis of multi-band spectral data, including radio, optical, and X-ray frequencies, to refine their model and affirm the presence of a structured jet.
Results and Interpretation
A pivotal outcome of the research was the rejection of the choked jet or cocoon model, which postulates that a jet fails to pierce through the surrounding ejected material, dispersing its energy in a spherical expanse. Instead, the data support a successfully launched structured jet with an angularly stratified energy distribution. The detection of a compact radio source and an expected displacement of 2.67 ± 0.3 milliarcseconds further corroborated their conclusion.
Implications
From a theoretical standpoint, the evidence of a structured jet's emergence from a binary neutron star merger imposes constraints on models of short gamma-ray bursts (sGRBs), suggesting at least 10% of such mergers successfully form relativistic jets. This has significant implications for understanding the typical structures and variability of sGRBs as well as aligning observed luminosity functions with theoretical predictions.
Future Directions
The confirmation of structured jets necessitates further exploration into their angular and energy profiles. Simulations and refined modeling could delineate the beaming effects and temporal evolution of such jets. A focused examination of their interaction with the interstellar medium (ISM) may offer insights into jet dynamics, energy conservation, and relativistic effects. Furthermore, the implications of this finding could extend to multi-messenger astronomy, integrating gravitational wave signals with electromagnetic observations for a comprehensive view.
Conclusion
The detection and characterization of a structured relativistic jet from the GW170817 event propels the understanding of binary neutron star mergers and their aftermath. As such, this augments the theoretical framework associated with jet dynamics and precise astrophysical phenomena identification, while providing a template for interpreting similar astronomical events in the future.
