Dependence of postmerger properties on the thermal heating efficiency in neutron star mergers

We systematically perform numerical-relativity simulations for equal-mass binary neutron star mergers for the models varying the thermal index <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi mathvariant="normal">Γ</a:mi><a:mrow><a:mi>th</a:mi></a:mrow></a:msub></a:math> with three different equations of state (EOSs) of the neutron stars (NSs), which are consistent with current multimessenger observational data and state-of-the-art theoretical calculations, and two different binary total mass (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msub><d:mi>m</d:mi><d:mn>0</d:mn></d:msub><d:mo>=</d:mo><d:mn>2.7</d:mn></d:math> and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mn>2.9</f:mn><f:msub><f:mi>M</f:mi><f:mo stretchy="false">⊙</f:mo></f:msub></f:math>). By varying the value of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi mathvariant="normal">Γ</i:mi><i:mrow><i:mi>th</i:mi></i:mrow></i:msub></i:math> within the hybrid EOS framework, we investigate the thermal effects on the merger dynamics, gravitational waves (GWs), and the dynamical mass ejection process. We find that the choice of the constant <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msub><l:mi mathvariant="normal">Γ</l:mi><l:mrow><l:mi>th</l:mi></l:mrow></l:msub></l:math> can change the outcome of the remnant for specific EOSs and <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>m</o:mi><o:mn>0</o:mn></o:msub></o:math>. We also show that the dynamical ejecta mass is affected by the <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msub><q:mi mathvariant="normal">Γ</q:mi><q:mrow><q:mi>th</q:mi></q:mrow></q:msub></q:math> value in a different way for different EOSs: for a stiff EOS the ejecta mass is high when <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:msub><t:mi mathvariant="normal">Γ</t:mi><t:mrow><t:mi>th</t:mi></t:mrow></t:msub></t:math> is small, while for softer EOSs the largest ejecta is achieved when <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mrow><w:msub><w:mrow><w:mi mathvariant="normal">Γ</w:mi></w:mrow><w:mrow><w:mi>th</w:mi></w:mrow></w:msub><w:mo>=</w:mo><w:mn>1.3</w:mn><w:mi>–</w:mi><w:mn>1.4</w:mn></w:mrow></w:math>. While the inspiral motion does not depend on the <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:msub><z:mi mathvariant="normal">Γ</z:mi><z:mrow><z:mi>th</z:mi></z:mrow></z:msub></z:math> value, the postmerger phase evolution is highly affected by that. We show that the dominant peak frequency <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:msub><cb:mi>f</cb:mi><cb:mn>2</cb:mn></cb:msub></cb:math> of the postmerger GW spectrum monotonically decreases as the <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"><eb:msub><eb:mi mathvariant="normal">Γ</eb:mi><eb:mrow><eb:mi>th</eb:mi></eb:mrow></eb:msub></eb:math> increases. We find that the universal relations between NS macroscopic properties and postmerger GW frequencies are subject to non-negligible thermal uncertainties, which can obscure the universal relation between the tidal deformability and <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:msub><hb:mi>f</hb:mi><hb:mn>2</hb:mn></hb:msub></hb:math>.