Nonparametric inference of the neutron star equation of state from gravitational wave observations

We develop a nonparametric method for inferring the universal neutron star (NS) equation of state (EOS) from gravitational wave (GW) observations. Many different possible realizations of the EOS are generated with a Gaussian process conditioned on a set of nuclear-theoretic models. These synthetic EOSs are causal and thermodynamically stable by construction, span a broad region of the pressure-density plane, and can be selected to satisfy astrophysical constraints on the NS mass. Associating every synthetic EOS with a pair of component masses M1,2 and calculating the corresponding tidal deformabilities Λ1,2, we perform Monte Carlo integration over the GW likelihood for M1,2 and Λ1,2 to directly infer a posterior process for the NS EOS. We first demonstrate that the method can accurately recover the properties of an injected GW signal, and subsequently use it to analyze data from GW170817, finding a canonical deformability of Λ1.4=160-113+448 and p(2ρnuc)=1.35-1.2+1.8×1034 dyn/cm2 for the pressure at twice the nuclear saturation density at 90% confidence, in agreement with previous studies, when assuming a loose EOS prior. With a prior more tightly constrained to resemble the theoretical EOS models, we recover Λ1.4=556-172+163 and p(2ρnuc)=4.73-2.5+1.4×1034 dyn/cm2. We further infer the maximum NS mass supported by the EOS to be Mmax=2.09-0.16+0.37 (2.04-0.002+0.22) M⊙ with the loose (tight) prior. The Bayes factor between the two priors is BIA≃1.12, suggesting that neither is strongly preferred by the data and that constraints on the EOS from GW170817 alone may be sensitive to the choice of prior.

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27 pages, 12 figures; references added

General relativity, alternative theories of gravity
saturation: density
equation of state
gravitational radiation
gravitational radiation: direct detection
gravitational radiation: emission
neutron star
statistical analysis
Monte Carlo
causality

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