Evidence for quark-matter cores in massive neutron stars. (arXiv:1903.09121v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Annala_E/0/1/0/all/0/1">Eemeli Annala</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gorda_T/0/1/0/all/0/1">Tyler Gorda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kurkela_A/0/1/0/all/0/1">Aleksi Kurkela</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nattila_J/0/1/0/all/0/1">Joonas N&#xe4;ttil&#xe4;</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vuorinen_A/0/1/0/all/0/1">Aleksi Vuorinen</a>

The theory governing the strong nuclear force, Quantum Chromodynamics,
predicts that at sufficiently high energy densities hadronic nuclear matter
undergoes a deconfinement transition to a new phase of quarks and gluons.
Although this has been observed in ultrarelativistic heavy-ion collisions, it
is currently an open question whether quark matter exists inside neutron stars.
By combining astrophysical observations and theoretical ab-initio calculations
in a model-independent way, we find that the inferred properties of matter in
the cores of neutron stars with mass corresponding to 1.4 solar masses are
compatible with nuclear model calculations. However, the matter in the interior
of maximally massive, stable neutron stars exhibits characteristics of the
deconfined phase, which we interpret as evidence for the presence of
quark-matter cores. For the heaviest reliably observed neutron stars with
masses of about two solar masses, the presence of quark matter is found to be
linked to the behaviour of the speed of sound c_s in strongly interacting
matter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive
neutron stars are predicted to have sizable quark-matter cores. This finding
has important implications for the phenomenology of neutron stars, and affects
the dynamics of neutron star mergers with at least one sufficiently massive
participant.

The theory governing the strong nuclear force, Quantum Chromodynamics,
predicts that at sufficiently high energy densities hadronic nuclear matter
undergoes a deconfinement transition to a new phase of quarks and gluons.
Although this has been observed in ultrarelativistic heavy-ion collisions, it
is currently an open question whether quark matter exists inside neutron stars.
By combining astrophysical observations and theoretical ab-initio calculations
in a model-independent way, we find that the inferred properties of matter in
the cores of neutron stars with mass corresponding to 1.4 solar masses are
compatible with nuclear model calculations. However, the matter in the interior
of maximally massive, stable neutron stars exhibits characteristics of the
deconfined phase, which we interpret as evidence for the presence of
quark-matter cores. For the heaviest reliably observed neutron stars with
masses of about two solar masses, the presence of quark matter is found to be
linked to the behaviour of the speed of sound c_s in strongly interacting
matter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive
neutron stars are predicted to have sizable quark-matter cores. This finding
has important implications for the phenomenology of neutron stars, and affects
the dynamics of neutron star mergers with at least one sufficiently massive
participant.

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