Inferring the Physics of Jets from Neutron Star-Black Hole Mergers Using Gravitational Waves

Core Concepts

Scientists can infer key properties of jets launched from neutron star-black hole mergers, such as the minimum remnant mass required for jet launching and the maximum viewing angle for detection, by analyzing gravitational wave data from future observatories like A#.

Abstract

• Bibliographic Information: Clarke, T. A., Lasky, P. D., & Thrane, E. (2024). Inferring jet physics from neutron star—black hole mergers with gravitational waves. arXiv preprint arXiv:2411.07035v1.

• Research Objective: This research paper investigates the possibility of using gravitational wave observations from future detectors, specifically A#, to determine the minimum remnant mass required for jet launching (Mmin) and the maximum viewing angle for GRB detection (θmax) in neutron star-black hole (NSBH) mergers.

• Methodology: The authors simulated a population of 200 NSBH mergers with parameters based on the recent GW230529 event. They injected these simulated signals into A#-like noise and performed parameter estimation using Bayesian inference. Assuming a top-hat jet model, they then applied a hierarchical inference framework to infer Mmin and θmax from the recovered gravitational wave parameters and simulated GRB detection outcomes.

• Key Findings: The study found that with 200 NSBH observations from A#, Mmin can be constrained to within 0.01 solar masses and θmax to within 13 degrees, assuming a fiducial neutron star equation of state and optimistic jet parameters.

• Main Conclusions: The authors conclude that future gravitational wave observations from A# have the potential to provide valuable insights into the physics of NSBH jets, even with a limited number of multi-messenger detections.

• Significance: This research is significant because it proposes a novel method for studying NSBH jet physics using gravitational waves alone. It highlights the potential of future detectors like A# to unravel the mysteries surrounding jet launching mechanisms and GRB production in these extreme cosmic events.

• Limitations and Future Research: The study acknowledges several limitations, including the simplified top-hat jet model and the assumption of a known neutron star equation of state. Future research could explore more realistic jet structures, incorporate uncertainties in the equation of state, and account for potential contamination from other astrophysical sources.

Stats

A simulated population of 200 NSBH mergers was used, consistent with the expected detection rate of A# in one year.
The minimum remnant mass for jet launching was set to 0.03 solar masses, and the maximum viewing angle for GRB detection was set to 35 degrees.
Only 5 out of the 200 simulated NSBH mergers resulted in detectable GRBs, given the chosen jet parameters.

Quotes

"Tidal disruption alone is probably not sufficient for the system to launch a jet and for the jet to be detected on Earth as a GRB."
"The strict physical requirements for disruption means that disrupting binaries are likely a small minority of NSBH systems."
"We show that we can resolve the minimum remnant mass to launch jets and constrain the maximum jet viewing angle with a population of NSBH mergers observed by A♯ in one year, provided some mergers have a multi-messenger counterpart."

Key Insights Distilled From

Inferring jet physics from neutron star - black hole mergers with gravitational waves

by Teagan A. Cl... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2411.07035.pdf

Deeper Inquiries

How might the use of more realistic jet models, beyond the simplified top-hat model, impact the accuracy of inferring jet parameters from gravitational wave data?

Employing more realistic jet models, which move beyond the simplified top-hat model, would significantly impact the accuracy of inferring jet parameters like minimum remnant mass (Mmin) and maximum viewing angle (θmax) from gravitational wave data. Here's how:

Increased Uncertainty: Realistic jet models, such as structured jets (e.g., Gaussian or power-law profiles), introduce additional parameters to characterize the jet structure and luminosity distribution. Marginalizing over these extra parameters during inference would inevitably broaden the posteriors for Mmin and θmax, leading to larger uncertainties in their inferred values.

Complex Correlations: The relationship between jet parameters and observables like GRB brightness becomes more intricate with realistic models. This complexity can introduce correlations between the inferred parameters, making it harder to disentangle the individual contributions of Mmin and θmax.

Model Dependence: The choice of a specific realistic jet model introduces model dependence into the analysis. Different models might yield varying constraints on Mmin and θmax, even when using the same gravitational wave data. This highlights the importance of exploring multiple plausible jet models and assessing the robustness of the inferred parameters.

Computational Cost: Realistic jet models often involve more computationally expensive calculations compared to the simple top-hat model. This increased computational burden could limit the exploration of the parameter space during inference, potentially affecting the accuracy of the results.
In summary, while realistic jet models offer a more accurate representation of the underlying physics, they come at the cost of increased uncertainty, complex correlations, model dependence, and computational challenges. Therefore, it's crucial to carefully consider these trade-offs when interpreting gravitational wave observations to infer jet parameters.

Could the presence of a dense environment surrounding the NSBH merger significantly alter the conditions required for jet launching and GRB production, and how would this affect the interpretation of gravitational wave observations?

Yes, a dense environment surrounding an NSBH merger can significantly alter the conditions for jet launching and GRB production, thereby impacting the interpretation of gravitational wave observations. Here's how:

Jet Choking: A dense circummerger medium, such as a thick accretion disk or a stellar wind from a companion star, can impede the jet's propagation. This "jet choking" effect can prevent the jet from breaking out and producing a detectable GRB, even if the conditions within the merger remnant are favorable for jet formation.

Modified Energy Requirements: The interaction between the jet and the surrounding medium can alter the energy requirements for a successful GRB. A denser environment might necessitate a more energetic jet or a longer-lived engine to overcome the resistance and produce a detectable GRB.

Altered Timescales: The presence of a dense medium can affect the timescales associated with jet propagation and GRB emission. The interaction can delay the emergence of the jet, leading to a longer delay between the gravitational wave signal and the GRB.

Impact on Viewing Angle: The interaction with the surrounding medium can also influence the jet's collimation and direction, potentially affecting the observed GRB viewing angle. This could lead to a discrepancy between the inferred viewing angle from gravitational waves and the actual angle from which the GRB is observed.
Impact on Interpretation of Gravitational Waves:

Underestimation of Jet Launching Rate: If jet choking is prevalent, the observed rate of GRBs associated with NSBH mergers might be significantly lower than the actual rate of jet launching events. This could lead to an underestimation of the fraction of NSBH mergers that produce jets.

Bias in Inferred Parameters: The modified energy requirements and altered timescales in a dense environment could bias the inferred values of Mmin and θmax. For instance, if jet choking is common, the inferred Mmin might be higher than the actual threshold for jet launching in a less dense environment.
In conclusion, considering the impact of the circummerger environment is crucial for accurately interpreting gravitational wave observations in the context of jet launching and GRB production. Future studies should incorporate detailed modeling of the environment and its interaction with the jet to refine our understanding of these powerful events.

If future observations consistently reveal NSBH mergers with no associated GRBs, what alternative theoretical frameworks could explain the absence of these powerful electromagnetic counterparts?

If future observations consistently show NSBH mergers without associated GRBs, it would challenge our current understanding of jet launching mechanisms and necessitate exploring alternative theoretical frameworks. Here are some possibilities:
1. Intrinsically Weak or Failed Jets:

Low Magnetic Fields: The strength of the magnetic fields threading the accretion disk plays a crucial role in launching and collimating jets. If the magnetic fields are too weak, the jet might be too weak to break out of the surrounding material or produce a detectable GRB.

Inefficient Energy Extraction: The process of extracting rotational energy from the black hole or the accretion disk to power the jet might be less efficient than currently assumed. This could result in jets that are too faint or short-lived to power a GRB.
2. Unfavorable Geometry or Viewing Angle:

Off-Axis Jets: Even if jets are launched, they might not be aligned with our line of sight. If the jet is pointed away from Earth, we would not observe a GRB, even if it is intrinsically bright.

Highly Collimated Jets: The jets could be extremely narrow, limiting the probability of observing them from Earth. If the jet opening angle is very small, only a tiny fraction of NSBH mergers would result in a detectable GRB.
3. Suppression by Baryon Pollution:

Choked Jets: As mentioned earlier, a dense environment surrounding the merger can choke the jet, preventing it from escaping and producing a GRB. This scenario is particularly relevant if NSBH mergers occur in regions with a high density of gas and dust.

"Dirty Fireballs": The merger ejecta might be rich in baryons (protons and neutrons), which can mix with the jet and reduce its Lorentz factor. This "baryon pollution" can suppress the GRB emission, making it too faint to be detected.
4. Alternative Energy Release Mechanisms:

Quasi-spherical Outflows: Instead of collimated jets, the merger remnant might release energy through more isotropic outflows, such as winds or cocoons. These outflows might not produce the characteristic high-energy emission of a GRB.

Engine Shutdown: The central engine powering the jet might shut down prematurely due to factors like fallback of material or changes in the accretion disk structure. This could result in a truncated GRB that is difficult to detect.
Further Investigations:
Distinguishing between these scenarios will require a combination of:

Increased Sample Size: Observing a larger population of NSBH mergers will provide better statistics and help constrain the rate of GRB association.

Multi-Messenger Observations: Combining gravitational wave data with observations across the electromagnetic spectrum (e.g., X-rays, radio) can provide clues about the presence and properties of jets, even if a GRB is not detected.

Improved Theoretical Models: Developing more sophisticated models of jet launching, propagation, and emission will be crucial for interpreting the observational data and understanding the conditions under which GRBs are produced or suppressed.