News

Possible detection of a kilonova afterglow

X-ray light curve from GW170817

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.

See the PSU press release and the arXiv preprint.

Prompt Collapse and new methodology for constraints on maximum mass of Neutron Stars

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. 

 

Figure showing the constant contours of the radius of maximum mass NS. The shaded regions are ruled out due to various constraints — (1.) Maximum mass must be greater than equal to heaviest pulsar observed so far with mass 2.01 solar mass. (2.) Maximum mass from phenomenological constraints comes out to be less than or equal to 2.9 solar mass. (3.) For every value of maximum mass, there exists a minimum and maximum value of radii. (4.) GW170817 was a delayed collapse hence threshold mass must be bigger than its total mass. The minimum and maximum values of maximum mass NS radius are shown in black dashed contours at the edge of the white region.
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. 
Please read full details here — https://arxiv.org/abs/2111.05183  and follow here for more of my research highlights.

Nuclear Physics from Multi-Messenger Mergers (NP3M) Focus Research Hub

Volume rendering of density and quark fraction in a binary neutron star merger.
Numerical simulation of a binary neutron star merger. The gray and blue contours correspond to densities of 10^13 and 10^14 grams per cubic centimeters, respectively. The red spots mark regions where our simulations predict that deconfined quark matter could appear.

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).

For more information, see the Penn State press release.

The new hub’s webpage is here.

QCD Phase Transitions in Binary Neutron Star Mergers

The appearance of deconfined quark matter (shown in red) during the postmerger evolution of a binary neutron star merger.

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.