Constraining the Maximum Mass of Neutron Stars From Multi-Messenger Observations of GW170817 (1710.05938v2)

Abstract: We combine electromagnetic (EM) and gravitational wave (GW) information on the binary neutron star (NS) merger GW170817 in order to constrain the radii $R_{\rm ns}$ and maximum mass $M_{\rm max}$ of NSs. GW170817 was followed by a range of EM counterparts, including a weak gamma-ray burst (GRB), kilonova (KN) emission from the radioactive decay of the merger ejecta, and X-ray/radio emission consistent with being the synchrotron afterglow of a more powerful off-axis jet. The type of compact remnant produced in the immediate merger aftermath, and its predicted EM signal, depend sensitively on the high-density NS equation of state (EOS). For a soft EOS which supports a low $M_{\rm max}$, the merger undergoes a prompt collapse accompanied by a small quantity of shock-heated or disk wind ejecta, inconsistent with the large quantity $\gtrsim 10^{-2}M_{\odot}$ of lanthanide-free ejecta inferred from the KN. On the other hand, if $M_{\rm max}$ is sufficiently large, then the merger product is a rapidly-rotating supramassive NS (SMNS), which must spin-down before collapsing into a black hole. A fraction of the enormous rotational energy necessarily released by the SMNS during this process is transferred to the ejecta, either into the GRB jet (energy $E_{\rm GRB}$) or the KN ejecta (energy $E_{\rm ej}$), also inconsistent with observations. By combining the total binary mass of GW170817 inferred from the GW signal with conservative upper limits on $E_{\rm GRB}$ and $E_{\rm ej}$ from EM observations, we constrain the likelihood probability of a wide-range of previously-allowed EOS. These two constraints delineate an allowed region of the $M_{\rm max}-R_{\rm ns}$ parameter space, which once marginalized over NS radius places an upper limit of $M_{\rm max} \lesssim 2.17M_{\odot}$ (90\%), which is tighter or arguably less model-dependent than other current constraints.

• The paper integrates gravitational wave and electromagnetic observations from GW170817 to derive an upper limit of ~2.17 solar masses for neutron stars.
• It employs a combined analysis of ejecta energy from kilonova and GRB jet data to constrain the high-density nuclear equation of state.
• The findings imply a softer EOS with smaller neutron star radii, guiding future multi-messenger astrophysical studies.

Constraining the Maximum Mass of Neutron Stars From Multi-Messenger Observations of GW170817

The paper of neutron stars (NSs), particularly concerning their masses and radii, is pivotal for understanding the high-density equation of state (EOS) of nuclear matter. The paper, authored by Ben Margalit and Brian D. Metzger, utilizes the multi-messenger observations from the event GW170817 to derive constraints on the maximum mass ($M_{\rm max}$) and radii ($R_{\rm ns}$) of NSs by integrating both electromagnetic (EM) and gravitational wave (GW) data.

Overview

The NS merger event GW170817 marked a significant advance in the multi-messenger astronomy era, with GW observations analyzed in conjunction with EM counterparts such as weak gamma-ray bursts (GRBs), kilonova (KN) emissions, and X-ray/radio signals suggesting a synchrotron afterglow. Notably, the type of remnant formed post-merger—whether a black hole (BH), supramassive NS (SMNS), or hypermassive NS (HMNS)—is sensitive to the NS high-density EOS.

Methodology and Findings

The paper proposes that GW170817 did not result in a prompt collapse to a BH, which would have been accompanied by a relatively small amount of ejecta, inconsistent with over $10^{-2}M_{\odot}$ of lanthanide-poor ejecta inferred from the KN. Instead, it suggests the possibility of a short-lived SMNS or HMNS, which would not exceed $M_{\rm max}$ sufficiently large to offset constraints posed by the observation of both blue and red KN ejecta with distinct velocities.

Interestingly, the paper integrates constraints on GW-inferred total binary mass with upper energy limits derived from EM signals, such as GRB jet energy ($E_{\rm GRB}$) and KN ejecta energy ($E_{\rm ej}$). This approach defines an allowed region in the $M_{\rm max}-R_{\rm ns}$ parameter space, resulting in an upper limit of $M_{\rm max} \lesssim 2.17M_{\odot}$ at a 90% confidence level. This estimate is particularly remarkable when compared to other constraints, which are generally more model-dependent.

Implications and Future Prospects

The constraints on $M_{\rm max}$ have crucial implications for our understanding of nuclear matter EOS in regimes beyond terrestrial experiments. The tighter upper limit on $M_{\rm max}$ implies a relatively softer EOS. This result contributes to a growing body of evidence, suggesting smaller NS radii, consistent with both tidal deformability constraints from GW and direct blue KN observations indicating compactness limits.

Prospective observations of BNS mergers could refine these estimates further, especially when higher-mass systems or more distinctive EM counterparts are encountered. Such discoveries may provide tighter lower limits on $M_{\rm max}$, particularly in cases of prompt collapses, and more pronounced upper constraints by observing instances that could lead to the formation of a long-lived SMNS or a stable NS.

Conclusion

Margalit and Metzger's paper on GW170817 effectively delineates constraints on the properties of NSs, marrying GW and EM data to address lingering challenges regarding NS EOS. While acknowledging the limitations inherent in current observations, this work provides a valuable and rigorous analysis that sets the stage for future explorations in the field of nuclear astrophysics, underscoring the potential of multi-messenger astronomy to unveil the inner workings of some of the universe's densest objects.