Tight multimessenger constraints on the neutron star equation of state from GW170817 and a forward model for kilonova light-curve synthesis

Abstract

We present a rapid analytic framework for predicting kilonova light curves following neutron star (NS) mergers, where the main input parameters are binary-based properties measurable by gravitational wave detectors (chirp mass and mass ratio, orbital inclination) and properties dependent on the nuclear equation of state (tidal deformability, maximum NS mass). This enables synthesis of a kilonova sample for any NS source population, or determination of the observing depth needed to detect a live kilonova given gravitational wave source parameters in low latency. We validate this code, implemented in the public mosfit package, by fitting it to GW170817. A Bayes factor analysis overwhelmingly (B > 1010) favours the inclusion of an additional luminosity source in addition to lanthanide-poor dynamical ejecta during the first day. This is well fit by a shock-heated cocoon model, though differences in the ejecta structure, opacity or nuclear heating rate cannot be ruled out as alternatives. The emission thereafter is dominated by a lanthanide-rich viscous wind. We find the mass ratio of the binary is q = 0.92 ± 0.07 (90% credible interval). We place tight constraints on the maximum stable NS mass, MTOV $=2.17^{+0.08}_{-0.11}$ M⊙. For a uniform prior in tidal deformability, the radius of a 1.4 M⊙ NS is R1.4 ∼10.7 km. Re-weighting with a prior based on equations of state that support our credible range in MTOV, we derive a final measurement R1.4 $=11.06^{+1.01}_{-0.98}$ km. Applying our code to the second gravitationally-detected neutron star merger, GW190425, we estimate that an associated kilonova would have been fainter (by ∼0.7 mag at one day post-merger) and declined faster than GW170817, underlining the importance of tuning follow-up strategies individually for each GW-detected NS merger.

The Challenges Ahead for Multimessenger Analyses of Gravitational Waves and Kilonova: A Case Study on GW190425

“…Currently, the relativistic jet that is launched in some mergers (see, e.g., Ruiz et al 2021) is not taken into account in the model presented here. Neither is the interaction between the jet and the dynamical and disk wind ejecta, which can alter the kilonova light curve (Klion et al 2021;Nativi et al 2021;Nicholl et al 2021).…”

“…The methods in Coughlin et al (2019a) and Dietrich et al (2020) allow for additional uncertainty in the analysis, attributed to the uncertainty in the modeling of ejected material, by adding an unknown component to the kilonova. There have also been more general studies developing a Bayesian framework for multimessenger analysis, focusing on several parameters describing the binary progenitor system (e.g., Barbieri et al 2019;Breschi et al 2021a;Nicholl et al 2021). Coughlin et al (2019a), Dietrich et al (2020), and Barbieri et al (2019) also consider the short gamma-ray burst (GRB) signal (e.g., Goldstein et al 2017) associated with compact object mergers, which can help constrain the inclination angle of the system (e.g., Ryan et al 2020).…”

“…More recently, there have been two new Bayesian analyses frameworks developed by Breschi et al (2021b) and Nicholl et al (2021). In Breschi et al (2021b), the authors use a threecomponent model with angular dependence, first presented in Perego et al (2017), and perform a joint analysis of GW170817 and the associated kilonova.…”

“…In Breschi et al (2021b), the authors use a threecomponent model with angular dependence, first presented in Perego et al (2017), and perform a joint analysis of GW170817 and the associated kilonova. In Nicholl et al (2021), the authors use the public light-curve generation code in MOSFiT (Guillochon et al 2018) and divide the ejecta into two components. Additionally they allow for a third component arising from shock heated material by a GRB jet.…”

Target of Opportunity Observations of Gravitational Wave Events with Vera C. Rubin Observatory

“…Coulter et al 2017;Arcavi et al 2017;Tanvir et al 2017;Lipunov et al 2017;Soares-Santos et al 2017), and a radio (e.g., Alexander et al 2017;Hallinan et al 2017) and X-ray (e.g., Troja et al 2017;Margutti et al 2017) afterglow. The identification of an EM counterpart pro-patrick et al 2017;Pian et al 2017;Rosswog et al 2017;Smartt et al 2017;Rosswog et al 2018;Watson et al 2019;Kasliwal et al 2019), placing limits on the neutron star (NS) equation of state (Bauswein et al 2017;Margalit & Metzger 2017;Coughlin et al 2018Coughlin et al , 2019aAnnala et al 2018;Most et al 2018;Radice et al 2018;Lai et al 2019;Dietrich et al 2020;Nicholl et al 2021), and making independent measurements of the Hubble constant (Abbott et al 2017a,b;Guidorzi et al 2017;Hjorth et al 2017;Hotokezaka et al 2018;Coughlin et al 2020a,b;Dietrich et al 2020;Wang & Giannios 2021). We refer the reader to Nakar (2020) and Margutti & Chornock (2020) for recent reviews of the GW and EM observations of GW170817.…”