Parita Mehta

Neutron stars (NS) are exotic state of matter with the highest possible density and
compactness in the observable universe, making them valuable laboratories for
studying such an extreme state of matter. They are of high interest because they
play a crucial role in governing many astrophysical phenomena:
1. The neutron star-neutron star (BNS) and neutron star-black hole merger are the
primary sources of gravitational waves.
2. These systems are also thought to be the central engines of short gamma-ray
bursts.
3. They are potential sources for electromagnetic and neutrino emissions.
4. Their merger is the source of production for very heavy elements in the Universe
5. They are associated with numerous explosive, transient, and non-electromagnetic
events.
6. For testing GR in the strong-field regime.
7. Sources for studying multi-messenger Astronomy and gravitational wave
Cosmology.
These make them Einstien’s richest laboratory in the Universe to explore the
relationship between gravity, light, and matter, but the governing equation of state
(EOS) for such extreme state is unknown. Yet, we know that one of the most
important characteristics of a neutron star is, its maximum allowed mass and even
though it is the most important prediction of the general relativistic theory of stellar
structures, its limiting value is still an unclear picture from many decades. This poster reviews the study on possible neutron star mass and its stability before collapsing to blackhole.

Why maximum mass is important?
1. Equation of state of NS: Different equations of state allow different maximum masses for neutron stars and, thus, determine the dividing line between neutron stars and black holes. Knowing the maximum possible masses along with the radii can put strong constraints on their EOS made a substantial impact on our understanding of the composition and bulk properties of matter at densities higher than that of the atomic nucleus, a major unsolved problem.
2.The ‘mass-gap’: When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun (2.5 solar masses), and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap?
3.Determining the outcome of binary neutron star merger: Recent detection of gravitational waves (GW190814) is a possible blackhole-neutron star merger. One of the components was an object with the mass of 2.6solar masses, which is probably too heavy to be a neutron star and is, therefore, more likely to be a black hole. However, we can’t rule out the possibility that GW190814 contains an especially heavy neutron star, a scenario that would cause us to dramatically revise our estimates of the maximum possible neutron star mass.

Recent simulations of the BNS merger (by Baiotti et al. 2017) suggest that the outcome of the merger could be a prompt collapse or will have an intermediate meta-stable product which is a highly massive differentially rotating neutron star; before the final collapse to a stable object i.e. a black hole or a neutron star. Hence, estimating the maximum possible mass of this differentially rotating short-lived remnant is consequently an easier way to put constraints on the maximum life span of this intermediate remnant, estimating the mass of the final stable neutron star, constraining EOS, possibly solving the puzzle of the mass gap and ascertaining the expected gravitational wave signals.

Interesting results:
1. A noticeable result was the discovery of four ‘types’ of configurations that co-exist with each other.
2. The maximum mass and various other astrophysical parameters were calculated for all types
of differentially rotating neutron stars modelled by a Γ= 2 polytropic EOS in Gondek-Rosinska
et al 2017 for the first time. For this particular EOS, the maximum mass of differentially rotating
neutron stars was shown to depend not only on the Ã, but also on the type of the solution.
It was shown that the maximum mass is an increasing function of à for type A
solutions and a decreasing one for types B, C, and D.
3. This behavior was found also for other polytropic EOS (Studzińska et al 2016) and recently confirmed
by Espino & Paschalidis (2019) and by Szkudlarek et al. 2019 for a realistic EOS and
strange quark matter EOS respectively.