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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.