The binary neutron star merger GW170817 is still bright in the X-ray more than 4 years after the merger. Our team has participated to the analysis of recent Chandra observations which has been just published as a Letter on the Astrophysical Journal. Our analysis shows that the current emission agrees with the prediction from our merger simulations.
Binary neutron star (BNS) mergers are one of the most violent events in our universe and one of the most ubiquitous and important sources in current and future gravitational wave (GW) observatories. Observables from these events reveal properties of supranuclear matter inaccessible to any experiments yet possible on the Earth. Evolution of a binary neutron star merger produces, eventually, either a black hole or, a NS. On a short timescale as well, the post-merger system could remain in either a NS or promptly collapse to form a BH with a lesser amount of radioactive matter in the disk around it.
It is believed that such prompt collapse occurs when the total mass of the binary reaches a critical value, called threshold mass. The value of the threshold mass is currently unknown since we are still uncertain about the properties of NS interiors (called equation of state, EOS) with several proposals for it. In fact, one of the main objectives of this research direction is to constrain the EOS of nuclear matter at supranuclear densities. We extend the previous assumption about the relationship of threshold mass with compactness and mass of maximum mass NS for a given proposal of EOS. We perform a series of 227 simulations using the numerical relativity code, Whisky-THC for a range of twenty-three equations of state with varying approximations about the matter. We find the threshold value for total mass beyond which the system collapses to the blackhole promptly without any bounce.
Lower limits of radii of maximum mass and 1.6 solar mass NS were obtained using only the absolute upper phenomenological limit of maximum compactness. We use maximum mass-dependent constraints on NS compactness of maximum mass to derive upper as well as lower limits of the radius of maximum mass NS. In addition, we find that our constraints can be used in future detectors to constrain the NS maximum mass in a narrow range using the total mass of BNS events and their identification as prompt or delayed collapse. We repeat this method to find radii of 1.6 as well as 1.4 solar mass NS which provides an independent method to constrain the range of maximum masses.
Our group is now part of the leadership team of a new $3.25M NSF Focus Research Hub dedicated to address fundamental questions in nuclear physics using multi-messenger observations of merging neutron stars. The hub is led by the University of Tennessee, Knoxville, and will directly support Penn State, Syracuse University, Indiana University Bloomington, and the University of Houston over a period of five years. Other Penn State professors involved in this initiative are Ashley Villar (Assistant Professor of Astronomy & Astrophysics), Kathleen Hill (Director of Penn State’s Center for Science in the Schools), and B. S. Sathyaprakash (Elsbach Professor of Physics and Professor of Astronomy and Astrophysics).
In a recent study, we investigated the phenomenon of QCD phase transitions to deconfined quark matter in the context of binary neutron star mergers. We computed gravitational wave signatures of such a phase transition and explored its thermodynamic consequences. We found that binaries with equations of state supporting a quark phase become more compact and are more susceptible to collapsing to a black hole. We also computed electromagnetic signatures of QCD phase transitions and observed that a merger of binaries producing a remnant with a quark core can be dimmer as compared to exclusively hadronic mergers at early times i.e. from a few days to a few weeks following the merger. On the contrary, at late times, on the order of several months to years, the quark mergers can start to become more luminous.
Our group is one of the 13 members of the new Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Physics Frontier Center, funded by the National Science Foundation. The center, formally approved Aug. 10, will launch Sept. 1, 2020.