Nonparametric inference of neutron star composition, equation of state, and maximum mass with GW170817

Abstract

The detection of GW170817 in gravitational waves provides unprecedented constraints on the equation of state (EOS) of the ultra-dense matter within the cores of neutron stars (NSs). We extend the nonparametric analysis first introduced in Landry & Essick (2019), and confirm that GW170817 favors soft EOSs. We infer macroscopic observables for a canonical 1.4 M NS, including the tidal deformability Λ1.4 = 211 +312 −137 (491 +216 −181 ) and radius R1.4 = 10.86 +2.04 −1.42 (12.51 +1.00 −0.88 ) km, as well as the maximum mass for nonrotating NSs, Mmax = 2.064 +0.260 −0.134 (2.017 +0.238 −0.087 ) M , with nonparametric priors loosely (tightly) constrained to resemble candidate EOSs from the literature. Furthermore, we find weak evidence that GW170817 involved at least one NS based on gravitational-wave data alone (B NS BBH = 3.3 ± 1.4), consistent with the observation of electromagnetic counterparts. We also investigate GW170817's implications for the maximum spin frequency of millisecond pulsars, and find that the fastest known pulsar is spinning at more than 50% of its breakup frequency at 90% confidence. We additionally find modest evidence in favor of quark matter within NSs, and GW170817 favors the presence of at least one disconnected hybrid star branch in the mass-radius relation over a single stable branch by a factor of 2. Assuming there are multiple stable branches, we find a suggestive posterior preference for a sharp softening around nuclear density followed by stiffening around twice nuclear density, consistent with a strong first-order phase transition. While the statistical evidence in favor of new physics within NS cores remains tenuous with GW170817 alone, these tantalizing hints reemphasize the promise of gravitational waves for constraining the supranuclear EOS.

Parameter estimation for strong phase transitions in supranuclear matter using gravitational-wave astronomy

“…While the current analysis of GW170817 seems to disfavor NSs with too-large radii and tidal deformabilities, consistent with the appearance of phase transitions, the data from this single event is insufficient to conclusively answer this question; see, e.g., Ref. [27]. Many recent works have addressed the question of whether GWs allow us to constrain the existence of hybrid stars, i.e., NSs that explores strong phase transitions to exotic forms of matter in their cores, and in particular twin stars [29,30,55,56,59,61,68].…”

“…In this paper, we have presented a reliable way of searching for phase transitions in supranuclear matter using the inspiral waveform of binary NS mergers, which is successful if the resulting mass-radius relation has only one or multiple stable branches. In contrast to previous works, which calculated the preference for multiple stable branches to search for strong phase transitions [27,28,30], our approach searches for an extended segment with c S = 0, i.e., we do not explicitly assume a multiple-branch feature in the M-R curve.…”

“…Our analysis follows a similar approach as previous works [18,27,31]. The analysis consist of a two-stage process:…”

“…Numerous efforts have been made to extract information on the EOS from the GW signal of BNS mergers, see, e.g., Refs. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]; cf. Ref.…”

“…Furthermore, Refs. [27,28] looked for indications of phase transitions in GW data using a nonparametric inference approach to the EOS. By combining heavy pulsar observations, GW170817, and the recent NICER observation [5], a Bayes factor in favor of the presence of multiple stable branches of 1.8 ± 0.2 [28] was found.…”