The outflow structure of GW170817 from late time broadband observations (1801.06516v2)

Abstract: We present our broadband study of GW170817 from radio to hard X-rays, including NuSTAR and Chandra observations up to 165 days after the merger, and a multi-messenger analysis including LIGO constraints. The data are compared with predictions from a wide range of models, providing the first detailed comparison between non-trivial cocoon and jet models. Homogeneous and power-law shaped jets, as well as simple cocoon models are ruled out by the data, while both a Gaussian shaped jet and a cocoon with energy injection can describe the current dataset for a reasonable range of physical parameters, consistent with the typical values derived from short GRB afterglows. We propose that these models can be unambiguously discriminated by future observations measuring the post-peak behaviour, with slope -1.0 for the cocoon and -2.5 for the jet model.

The Outflow Structure of GW170817 from Late Time Broadband Observations

The paper by Troja et al. provides a comprehensive analysis of the outflow structure of GW170817, a significant event in the field of multi-messenger astrophysics. Utilizing a spectrum of observational data ranging from radio to hard X-rays, and incorporating results from the LIGO/VIRGO gravitational wave observations, the study offers a detailed comparison of jet and cocoon models for the relativistic outflows associated with this event.

Overview and Conclusions

The research primarily addresses whether the afterglow of GW170817 can be attributed to a structured jet or a cocoon with energy injection. Notably, it rules out simplistic models such as the homogeneous and power-law jets and simple cocoon models. Instead, both the Gaussian-structured jet and the cocoon with continuous energy injection align with the observational data within acceptable physical parameter ranges, reminiscent of typical short gamma-ray burst (sGRB) afterglows.

The authors propose that distinguishing between these models will be possible through future observations of post-peak behaviors. Specifically, they predict a decay rate of $F_\nu \propto t^{-1.0}$ for the cocoon model and $F_\nu \propto t^{-2.5}$ for the jet model. This distinctive divergence in decay rate offers a tangible metric for identifying the outflow's nature.

Numerical and Model Implications

Through Bayesian MCMC modeling, the paper estimates several key parameters of both models. For the Gaussian jet, results point towards a narrow core ($\theta_c \approx 0.091$ rad) and a substantial viewing angle ($\theta_v \approx 0.51$). In contrast, the cocoon model suggests an energy distribution dominated by slower ejecta, requiring energy injection to match observations.

The analysis benefits from the unique opportunity to integrate gravitational wave data into the modeling of electromagnetic observations. Incorporating constraints on viewing angles from LIGO/VIRGO enhances parameter accuracy and underlines the synergetic potential of multi-messenger astronomy.

Implications and Future Developments

The distinction between a structured jet and a cocoon has profound implications for our understanding of sGRBs. Should the Gaussian jet model prove accurate, it would reaffirm the notion that sGRBs arise from neutron star mergers, aligning GW170817 with standard sGRB characteristics when corrected for viewing angle. Conversely, a successful cocoon model could imply a broader range of phenomena arising from such events, indicating a potentially larger occurrence rate of observable kilonovae, albeit weaker in gamma-ray emission.

This research underscores the critical nature of continued observational efforts across the electromagnetic spectrum. By resolving the debated outflow structure, scientists could refine theoretical models of sGRB formation and enhance predictions for future multi-messenger detections.

Conclusion

Troja et al.'s study of GW170817 offers rich insights into the nature of relativistic outflows from compact binary mergers. The findings not only advance our comprehension of the event itself but also contribute to the broader dialogue on sGRB origins and multi-messenger astrophysics. Future observations will play a pivotal role in resolving the ambiguity between competing models and fostering a fuller understanding of these cosmic events.