Finite tidal effects in GW170817: Observational evidence or model assumptions?

Abstract

After the detection of gravitational waves caused by the coalescence of compact objects in the mass range of neutron stars, GW170817, several studies have searched for an imprint of tidal effects in the signal, employing different model assumptions. One important distinction is whether or not to assume that both objects are neutron stars and obey the same equation of state. Some studies assumed independent properties. Others assume a universal equation of state, and in addition that the tidal deformability follows certain phenomenological relations. An important question is whether the gravitational-wave data alone constitute observational evidence for finite tidal effects. All studies provide Bayesian credible intervals, often without sufficiently discussing the impact of prior assumptions, especially in the case of lower limits on the neutron-star tidal deformability or radius. In this article, we scrutinize the implicit and explicit prior assumptions made in those studies. Our findings strongly indicate that existing lower credible bounds are mainly a consequence of prior assumptions combined with information gained about the system's masses. Importantly, those lower bounds are typically not informed by the observation of tidal effects in the gravitational-wave signal. Thus, regarding them as observational evidence might be misleading without a more detailed discussion. Further, we point out technical problems and conceptual inconsistencies in existing studies. We also assess the limitations due to systematic waveform model uncertainties in a novel way, demonstrating that differences between existing models are not guaranteed to be small enough for an unbiased estimation of lower bounds on the tidal deformability. Finally, we propose strategies for gravitational-wave data analysis designed to avoid some of the problems we uncovered.

Neutron-star tidal deformability and equation-of-state constraints

“…In the following discussion of such studies it is worth keeping in mind that all analyses use the same data (modulo a recalibration between [29] and [31]), namely the gravitational wave signal for GW170817 2 . Therefore any differences between the results are solely due to the different prior each analysis employs to either link the component tidal deformabilities or to restrict to specific equation of state models [212,213]. Figure 7 summarizes these results.…”

Constraining the Dense Matter Equation of State with Joint Analysis of NICER and LIGO/Virgo Measurements

“…Constraints on the mass and radius have recently also been obtained from the gravitational wave (GW) observations of the binary NS merger event GW170817 (Abbott et al 2017). The LIGO/Virgo collaboration reported measurements of the masses and EOS-dependent tidal deformability parameters of the NSs under different prior assumptions on the spins and using various waveform models (Abbott et al 2019a,b); see Kastaun & Ohme (2019) for a critical re-examination of the results. The corresponding radii and EOS constraints were inferred in two ways, by using a parameterized spectral EOS (Lindblom 2010) and by employing EOS-insensitive relations, both using the low spin priors (cS/GM 2 < 0.05 where S is the spin angular momentum) and hence non-rotating stellar models.…”

Reanalysis of the binary neutron star mergers GW170817 and GW190425 using numerical-relativity calibrated waveform models

“…The constraint can be further improved by assuming the EOS to be common for both NS [16,17] (but see also Ref. [18]) as is also done in an independent analysis [19,20]. These constraints are used to investigate the NS EOS [21][22][23] as well as those for quark and hybrid stars [24][25][26].…”