Speed of sound in dense matter and two families of compact stars. (arXiv:2102.02357v2 [astro-ph.HE] UPDATED)

Speed of sound in dense matter and two families of compact stars. (arXiv:2102.02357v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Traversi_S/0/1/0/all/0/1">Silvia Traversi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Char_P/0/1/0/all/0/1">Prasanta Char</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pagliara_G/0/1/0/all/0/1">Giuseppe Pagliara</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Drago_A/0/1/0/all/0/1">Alessandro Drago</a>

The existence of massive compact stars $(Mgtrsim 2.1 M_{odot})$ implies
that the conformal limit of the speed of sound $c_s^2=1/3$ is violated if those
stars have a crust of ordinary nuclear matter. Here we show that, if the most
massive objects are strange quark stars, i.e. stars entirely composed of
quarks, the conformal limit can be respected while observational limits on
those objects are also satisfied. By using astrophysical data associated with
those massive stars, derived from electromagnetic and gravitational wave
signals, we show, within a Bayesian analysis framework and by adopting a
constant speed of sound equation of state, that the posterior distribution of
$c_s^2$ is peaked around 0.3, and the maximum mass of the most probable
equation of state is $sim 2.13 M_{odot}$. We discuss which new data would
require a violation of the conformal limit even when considering strange quark
stars, in particular we analyze the possibility that the maximum mass of
compact stars is larger than $2.5M_{odot}$, as it would be if the secondary
component of GW190814 is a compact star and not a black hole. Finally, we
discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration
compare with our analysis (not based on them) and with other possible
interpretations.

The existence of massive compact stars $(Mgtrsim 2.1 M_{odot})$ implies
that the conformal limit of the speed of sound $c_s^2=1/3$ is violated if those
stars have a crust of ordinary nuclear matter. Here we show that, if the most
massive objects are strange quark stars, i.e. stars entirely composed of
quarks, the conformal limit can be respected while observational limits on
those objects are also satisfied. By using astrophysical data associated with
those massive stars, derived from electromagnetic and gravitational wave
signals, we show, within a Bayesian analysis framework and by adopting a
constant speed of sound equation of state, that the posterior distribution of
$c_s^2$ is peaked around 0.3, and the maximum mass of the most probable
equation of state is $sim 2.13 M_{odot}$. We discuss which new data would
require a violation of the conformal limit even when considering strange quark
stars, in particular we analyze the possibility that the maximum mass of
compact stars is larger than $2.5M_{odot}$, as it would be if the secondary
component of GW190814 is a compact star and not a black hole. Finally, we
discuss how the new data for PSR J0740+6620 obtained by the NICER collaboration
compare with our analysis (not based on them) and with other possible
interpretations.

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